DVB-S on the spectrum analyzer

Here's what DVB-S looks like on a spectrum analyzer:



This is with the modulator configured for a 4500 kilobaud signal, which results in a 6 MHz wide channel. With a FEC rate of 3/4, that's about 6 MB/sec or so of available MPEG bandwidth.

Compared to 8VSB, the envelope isn't quite so square - it rounds off a bit on either end. And, of course, there is no pilot at the bottom end like with 8VSB.



This is another SA view - this time it's calculating the channel average power and the adjacent channel power. Taking into account the 20 dB attenuator on the input of the SA, it's seeing a channel average power of about 4 watts, and the adjacent channels are 30 dB down from there.

N6QQQ/R bench test results

The receiver and my uplink DVB-S modulator have arrived from Germany. With the DEMI 2330PATV, I'm able to get about 4 watts of DVB-S power output. When I started bench testing the repeater, however, it turned out that even with the amp's bias turned off, the DVB-S exciter alone was enough to be received by the receiver. Not too much of a surprise, given that the exciter and receiver were about 3 feet apart in the garage.

I made a YouTube video showing a complete round-trip - from my analog camera, through the MPEG encoder, modulated with DVB-S, amplified to 4 watts and transmitted with my 23 cm 14 dBd loop Yagi. Then received on the DVB-S receiver, modulated as 8VSB on 33 cm, amplified to about 15 watts, then attenuated by 20 dB and run through a rubber duck antenna. Then received on the loop Yagi on the roof, downconverted from there to TV channel 3, and then received on my portable ATSC TV.

Whew!



The next step will be to start separating the up/downlink station and the repeater further and further apart. Hopefully, the last test will be with the repeater at its new home on the hill!

The next big demo will be at the K6BEN ATV luncheon on November 23rd. I'll schlep all of the gear for both the repeater and my uplink station over to Harry's Hofbrau and we'll try a full round-trip across the parking lot.

PLL design

The North Country Radio downconverter kit arrived today. It's a bit daunting, but they did supply some suggestions on how to tap off the RF sample for the PLL and put in the VCO control voltage. That was nice of them.

It's hard to find PLL components anywhere - certainly there aren't any one stop shops anymore. Makes me wonder how companies prototype stuff like this in this day and age.

Jameco still sells the MC145151 in DIP packaging, which is convenient.

The North Country Radio downconverter schematic says that the VCO control voltage is 2 to 8 volts. That exceeds the 5 volts that run the MC145151 PLL, which means that the PLL loop filter will need to be an active one. Given that the reference frequency in this case is 15,625 Hz, the op amp for the active loop filter isn't going to have a lot asked of it - an LM741 would do just fine.

That just leaves a divide by 64 prescaler. That, it turned out, was hard to find, and impossible in DIP packaging. I wound up buying a µPB1507GV from Mouser, but I will need to solder it down to an SSOP-8 to DIP adapter board for prototyping.

The nice thing about working on PLLs is that only on the input side of the prescaler are you dealing with real RF - the entire rest of the circuit is dealing with low enough frequencies that it can all be breadboarded.

More PLL design

So the design at the moment for the downconverter looks like it'll use the MC145151 PLL and the SA620 LNA/Mixer/VCO. At first, I figured I'd use the MC12080 divide-by-80 prescaler. With a 12.8 MHz crystal and a divide by 1024 reference divisor, that's a channelization of 1 MHz. And that's fine for an ATV downconverter.

But what if we altered the design a bit to take advantage of the fractional capabilities of the a dual modulus PLL chip, like the MC145152?

If we used that same 12.8 MHz crystal and divide by 512, we'd have a 25 kHz reference frequency. With a divide-by-64/65 prescaler (like the MC12054A), we'd be able to achieve the same result, but with a 25 kHz channel step.

A PLL that can drive a dual modulus prescaler has two counters. One of them is the actual output into the phase comparator, the other causes a digital output to change state during each count cycle. That signal changes the prescaler from a divide by P+1 to a divide by P.

Let's say that we set the main counter value to N and the prescaler change counter to A and the lower of the two prescaler values to P. What we'd then wind up with is dividing the VCO output by P*N+A. If the reference frequency is R, then the output frequency will be R*(PN+A). So for 848 MHz, if P is 64, and R is 25 kHz, N would be set to 530 and A to 0. Set A to 1 and the output would instead be 848.025 MHz. For 849 MHz (which puts 909-915 down to TV channel 3), you set N to 530 and A to 40.

In general, for a desired frequency F, you set the N to the integral quotient of (F/R) divided by P. You then set A to (F/R) mod P. Stated another way, you're building a fraction of N + A/P, which winds up being equal to F/(P*R).

Of course, this does mean that the design will need more DIP switches. The MC145152 has 6 bits for the A value and 10 bits for the N value. That's two banks of 8 switches. You could fix the top 4 bits of N to 1000 - limiting N from 512 to 575. The resulting frequency range would be 819.2 MHz to 921.6 MHz - more than enough for our purposes. That's a total of 12 switches - two banks of 6.

So a 12.8 MHz crystal, an MC145152 PLL, and an MC12054A prescaler and an SA620 LNA/VCO/Mixer.

Now the big problem is either finding inventory on these parts somewhere or finding equivalents.

K6BEN retransmitted digitally

Today I retransmitted the output of the K6BEN repeater digitally over ATSC.



As soon as N6QQQ/R goes live, I'll start doing this on Wednesday evenings just to sort of bootstrap the use of the new repeater - that way folks will have more to look at on it than just my garage.

Designing a downconverter

I've done some looking into it, and it appears that the three parts that would be the most useful would be the SA620 LNA, VCO and mixer, an MC145151-2 DIP switch controlled PLL, and an MC12080 UHF prescaler. From what I can tell, those three parts should be able to work together to downconvert from just about any frequency in the 33 cm band down to VHF channel 3 or 4.

Yeah, I know. In the last post, I made an argument for putting it in UHF instead. But the coax losses are much, much lower at VHF, so if it's reasonable or easy to make it happen, well, why not?

The big problem is, so far as I can tell, the SA620 and MC12080 are only available as surface mount devices. It'd be nice if they were available in DIP packaging. But, alas, no. SparkFun electronics, however, sells SOIC-8 and SSOP-20 to DIP adapter boards.

The concept is that the RF comes in and the LNA built-in to the 620 amplifies it a little and feeds it into the mixer. Meanwhile, the VCO generates the IF of about 850 MHz or thereabouts. A little of the VCO's output is fed through the prescaler, and then out to the PLL. The PLL then generates corrections for the VCO's voltage.

Those 3 chips total are less than $15, from what I can tell, from a combination of DigiKey and Jameco.

And that's all there is to it: 3 chips, a 5 volt regulator, 2 sets of DIP switches and some discrete components to tie it all together. For extra credit, it would be fairly simple to add a bias-T to the output to supply power to the thing to mast-mount it. And if it's going to be mast-mounted, then DIP switches are a completely reasonable way to set the LO frequency, since it'll be something you only set once.

In fact, if you're using a computer peripheral tuner, like the HDHomerun, you'll likely just set it for an LO frequency of 848 MHz - that would put 902 MHz at 54 MHz. You'd then just tell your tuner exactly what frequency it should use. The only reason for making the PLL as adjustable as it is is because TV sets aren't agile enough to hunt around other than on normal TV channels. The HDHomeRun, however, can easily be told to look for 8VSB on a channel center of 64 MHz - which is where the 909-915 MHz ATV channel would be found.

Final repeater pieces ordered

All the pieces are coming together.

The repeater will consist of

From SR Systems:

1 DVB-S 23cm receiver board
1 ATSC 33cm minimod

From Downeast Microwave:

1 3370PAHS amplifier

From Ham Radio Outlet:

1 Diamond X6000A 2M/70cm/23cm vertical antenna
1 Comet KP-20 33cm vertical antenna

I'm going to also set up an uplink station for myself. I want to use it at least at first to retransmit the output from the K6BEN repeater. It will consist of

From SR Systems:

1 MPEG encoder board
1 DVB-S 23cm minimod

From Downeast Microwave:

1 2330PATV amplifier

From PC Electronics:

1 TVC-4S 70cm downconverter
1 RCV3 channel 3 NTSC demodulator

The concept here is that we receive standard 70 cm AM TV on the TVC-4S channel 1 down to baseband video and audio. Send that into the MPEG encoder and transmit it as DVB-S on 23cm up to the repeater. The repeater will then retransmit the transport stream over ATSC on 33cm.

I'll receive that with my receive station:

From PC Electronics:

1 TVC-9S 33cm downconverter
1 HDHomeRun ATSC tuner

I'm not sure I can configure the DVB-S modulator to key on and off based on whether or not there's good video going into the MPEG encoder. If it is, then I could make the whole thing automatic. But if my experience with the ATSC modulator is any guide, it won't do that, so I'll have to turn the thing on and off myself. But Stefan assures me that the repeater (the DVB-S NIM and minimod combination) can key on and off automatically based on the receive signal.

Downconverters for receiving

The PC Electronics TVC-9S is a very nice downconverter, and I'm glad I bought one, but it is a bit expensive. To really make it easy to get started, I think it would be great if something simpler were available.

North Country Radio make a VCO controlled 33 cm downconverter. I haven't tried it, though, and my one worry is that without a frequency counter, it might be tough to tune. It depends on how much tuning slop your ATSC tuner will allow. But on the other hand, their downconverter is a third the price of the PC Electronics one.

One thing I think that could be changed is that these downconverters are designed to shift 33 cm down to VHF low. That means their mix frequency must be very high - in the 800 MHz or so range.

For DTV, I don't think it's necessary to chose such a low IF. If you picked a mix frequency of, say, 421 MHz, you'd wind up with 909 MHz being output on TV channel 17. That would be ideal for around here, because channels 15 through 18 are unused (since 16 and 17 are used for land-mobile in San Francisco).

So, dear readers, if you can design a cheap crystalized or PLL controlled downconverter to go from the middle of 33 cm to the lower end of the UHF TV band, please let me know - I think you could make a lot of hams very happy.

We have a site!

I've negotiated a lease with a land owner in the East San Jose foothills, so as soon as I can get the last of the equipment, North America's first 100% digital ATV repeater will be on the air!

I've set up a a web site for it, with all of the information on what it will take to transmit in and receive from it.

Low bitrate MPEG2

I've been using 5 MB/sec for my experimental transmissions, but I have been hearing from Stefan that European hams have had success with 2 MHz wide 2 MB/sec DVB-S. So on a whim, I set my ATSC transmitter to transmit 2 MB/sec MPEG2 video. And the resulting video looked just fine to my eyes. It got a little blocky when I panned the camera around, so with high motion subject matter, it's not enough. But it's clearly enough for the average QSO.

I still think the receivers should be configured for as much bandwidth as the bandplans will allow (6 MHz on 23cm and 4 MHz on 70cm). If for no other reason than it's conceivable that HD pictures could be sent with H.264 inside 6 MB/sec, and someone might want to try that (despite the fact that it won't be compatible with ordinary TVs).

Diamond X6000A

Wow, the X6000A is big! Yeah, the specs say it's 10 feet long, but it just doesn't really come home until you put it together, I guess. It's a couple feet longer than the Comet KP-20, even. And it has much, much longer radials (likely because they have to work on 2 meters).

One of the issues will wind up being how to mount the two antennas. Of course, most of that depends on the site. But despite the cross-band nature of the repeater, it's still going to be desirable to keep the transmit and receive antennas away from each other - either at different elevations on a tower, or 10-20 feet away from each other horizontally. That will obviate the need to set up a 33 cm band-reject filter to protect the receivers.

Operational differences

Users of K6BEN/R will have gotten used to how that repeater works - in particular, the use of 2 meter audio for coordination. I used to think it odd that the audio wasn't carried into the repeater as part of the uplink, but having used the repeater, it's clear that it's a fairly useful way to set things up. Someone other than the station sending video can be the source for te audio.

The first iteration of the D-ATV repeater won't have this ability: it will simply retransmit the MPEG TS uplinked by the user station. This means that coordination will have to take place via a traditional 2m or 70cm repeater. One thing that is planned for the future, though, is a shack camera going into an MPEG encoder. But that still would mean it would be a different stream than the one the user is transmitting. So you could watch yourself on TV, or listen to the 2m audio and watch the shack cam, but not both (unless you either decode both at the same time with a computer or multiple TVs).

On the plus side, however, the audio that's part of the MPEG encoding will be vastly higher quality than the narrowband FM we're used to. That may be useful in the context of events. Certainly, the ability to multiplex multiple programs and (when multiple receivers come), uplink multiple simultaneous streams will be very useful for events.

Open colector output

Perhaps it's obvious to most electronics experts reading this, but I thought I'd pass along something I learned today.

As you all know, I'm trying to build a digital ATV repeater. One of the things I needed was a system to key the amp on and off so that it doesn't have to stay biased 24/7 (a waste of power, unnecessary heat and shortens the amp's service life). Fortunately, the amp has a short-to-ground PTT line.

Well, it turns out that the modulator has a TTL "PTT" pin on one of its headers. How do you turn this into a short-to-ground output?

The answer is open collector. I installed an RCA jack in the chassis and simply dead-bug soldered an NPN transistor to the jack, emitter to shell (and ground) and collector to center. I then soldered a 10K resistor to the base, and the other end to a wire to a "pin clip" to clip on to the PTT pin.

And it works perfectly! When I use the menu item to take the transmitter from stand-by to on-air, it keys the amp! Yay!

Gearing up

Next month, I'm going to order a DVB-S modulator (for me) and a DVB-S receiver for the repeater.

Stefan (from SR Systems) says that you can set up a repeater simply by connecting the receiver directly up to the modulator. The modulator will turn on automatically when the receiver detects good reception. Not only that, but it has a "PTT" logic indicator pin that can turn into an open collector output with one transistor and one resistor to key the amp!

After that's done, the MPEG encoder board will be paired with the new DVB-S transmitter board as my user station.

The bad news is that they don't make a DVB-S receiver for 70 cm. Too bad. We'll have to receive on 1.2 GHz. But, of course, we'll be able to do so in a 2 MHz wide channel so we should not have the same interference problems we do with FM TV on K6BEN/R.

All I need besides that is a 15A power supply for the whole thing, a 23 cm antenna, some coax.... and a place to put it all.

For the receive antenna, I'm looking at the Diamond F1230A. It has 13.8 dBi gain. With about 3 dB of coax loss and 9 dB of loss for operating on 1.2 GHz, you should be able to use the PC Electronics ATV DX chart as it is - without having to correct for being on 1.2 GHz. Just assume that the receive site has unity gain. So, for example, if you can generate 5 watts into a Directive Systems 14 element loop Yagi, you'd be able to hit the system from about 15 miles away.

Digital amateur TV for the rest of us

I've been concentrating on the ATSC repeater output all this time and have neglected considering the user uplink side of the equation. In a single e-mail with the SR Systems folks, I think I see the way forward.

NARCC, the local repeater coordination body here in Northern California, has not been friendly to ATV in the 70 cm band. And you can't really blame them - it's 6 MHz of real estate that is sorely needed for other things. The big problem is that generating power at 70 cm is vastly less expensive and easier than for the higher bands. Given that state of affairs, it would be very, very nice to be able to use 70 cm as a user uplink band for a DATV repeater.

Well, it turns out that according to Stefan, the europeans have had great success with DVB-S in a 2 MHz wide bandwidth at about 2 mbps encoding! That's excellent news! I'd be shocked if we couldn't find a 2 MHz channel somewhere between 420 and 440 MHz for this.

Not only that, but the DVB-S encoder board is less than half the price of an ATSC one. That means that a user station can be had for €590. Add to that the cost of a Downeast Microwave 7025PACK and you're on the air with about 8 watts of transmitter power for just over $1000! That's not bad for digital ATV!

ATSC Haiku

Blue screen vanishes
blue sky and green fields appear
call sign, blue screen returns

6 dB ratio
from peak to channel average
power costs a lot

no snow, no ghosting
perfect signal every time
except for drop-outs

Ricochet bankrupt
phoenix is reborn from ash
on 33 cm

MPEG2 for sound
AC-3 costs too damn much
stupid patent trolls

K6BEN
ATV serving all of
silicon valley

ATSC power conundrum solved?

I've learned a bit more about some of the features of the HP 8590L spectrum analyzer I got off eBay a while ago. One feature it has is the ability to calculate channel average power and the adjacent channel power ratio (that is, how many dB down the power is on the 6 MHz above and below the channel of interest). And at the preferred settings of the exciter, the output power is about 15 watts, with the adjacent channels being down about 35 dB. That's a little more out-of-channel garbage than I had hoped for - a bit more than 20 mW of ERP. But even so, on 900 MHz that small amount of power should die easily within half a mile of the site. Heck, I've transmitted with the bare exciter at approximately that power level and been unable to receive it half a block away from home. So I'm not going to worry about it. As always, reducing power will reduce the reconstituted sidebands, though the bare exciter has sidebands that are only about 48 dB down from the main signal, so there's only so much more improvement to be had.

Another bit of good news is that I spent some time "boiling the dummy load" this evening. Without the muffin fan, the heat sink of the amp gets so hot that you can't comfortably touch it. But with the muffin fan sitting on top and running, the heat sink stays almost as cool as the ambient air temperature (almost). So that bodes really well for the health of the amp when it's put in service.

Final TX configuration set

I've set up the transmitter in what I believe will be its final configuration. I put a 6 dB attenuator on the output of the exciter to raise the output power setting required so that it would be in the middle of its range rather than on the end. Getting the exciter output to an average power of about 6 dBm now means setting the output power to 10 instead of 3. I traded some mail with the guys at SR-Systems and they said that the "sweet spot" is anywhere between 5 and 13. Anyway, with 6 dBm of power, the amp is supposed to generate 15 watts of power, which means an output ERP of about 75 watts. I put an N to UHF adapter in the input of the spectrum analyzer and then put a big nail in the center conductor socket and laid the coax next to it that was hooked up between the amp and the dummy load. It may be less than perfectly accurate, but my hope is that it's close enough to score. Anyway, with that setup, I could see the "ramps" of out-of-channel noise well enough to check how strong they were relative to the ATSC pilot (which, again, is 11.3 dB down from the channel average power). At a power setting of 10, the skirts were just about 47 dB down from the average power, which is right where the part 73 channel mask would have them. At a power level of 11, they come up to closer to 40 dB down. Now, the part 73 channel mask is not of any particular relevance for part 97 operations, but we amateurs are required to reduce spurious emissions in accordance with good engineering practice. Besides, limiting the output power can't help but make the amplifier just that much happier, which means it is likely to last longer - particularly given that it will be in a repeater shack that is not climate controlled.

I have a 20 watt 20 dB attenuator arriving in the mail tomorrow, so I'll be able to run the output of the amp directly into the SA for a final check tomorrow evening. Not only will I be able to directly measure the out-of-channel emissions, but I'll be able to get an accurate power output reading as well.

So it will put out 75 watts of ERP (perhaps it might drop down to 70 or so with coax losses when all is said and done). That still should be receivable everywhere the analog signal of K6BEN is receivable today.

Get your popcorn ready

So, the 33 cm ATSC transmitter will hopefully be going into more or less full time service sometime soon now.

So just to get people prepared, let me outline what you'll need in order to see the transmissions.

First and foremost, you must have a line-of-sight to the east San Jose foothills. And it has to be a real line of sight. That means that on a clear day you can look in the direction of Mount Hamilton and see the mountains. A single tree in the way will knock your received signal strength down by 20 dB - 100 times!

At the location where you have that nice, clear line of sight towards Mount Hamilton, you'll need to mount a 900 MHz directional antenna. I'm about 10 miles away from where the transmitter is going to be located, and I can receive it with a 7 element 10 dBi 900 MHz Yagi. I bought it off the Internet for about $30. If you're further away, or if you have trees in the way, you'll need something bigger, like the Directive Systems 3318LYARM 18 element 16.5 dBi loop Yagi.

You'll need to use as short a run of coax as possible from the antenna to the downconverter, and the best coax you can afford. Losses at 900 MHz can be severe. I use LMR-400.

The best downconverter I've found can be had from P.C. Electronics. You'll want the TVC-9S 900 MHz PLL controlled downconverter. From there, you can run ordinary 75 ohm cable TV coax from the downconverter to your ATSC tuner or TV. Set the downconverter to channel 2, which maps 910-916 MHz to TV channel 3.

As a bonus, if you have a good clear line of sight to Mt. Diablo and an analog TV, you can try picking up the W6CX analog ATV repeater output on 918-924 MHz.

You'll need to perform a channel scan while the transmitter is active in order to discover the downconverted signal on channel 3. Fortunately, the K6BEN repeater is active from 0 to 10 minutes past each hour transmitting ID slides. So your best bet is to scan at about 2 minutes past the hour. Of course, if your tuner/TV is capable of tuning directly to a channel without scanning, you can simply tune it to channel 3.

New Amateur ATSC DX record - complete with video

I went up to the Sierra Rd QTH in the east foothills, and this time everything went perfectly. I was able to capture a video from up there, and with a good line of sight and 80 watts of ERP, I was able to get a signal strength of 100 and a signal quality of 75 or so on the HD HomeRun. This location is very similar to the location of the K6BEN repeater and is in, more or less, the same direction. Google Earth says it's 9.07 miles away from home. The coordinates are 37.413081° N, 121.824316° W.



Here's a screenshot from my phone's ssh client showing the status report from the HDHomeRun with the signal strength and qualty (the first line showing no reception was a control - the transmitter was unkeyed):



It was actually sort of tough to get the phone to cooperate. Cell phones operate in the 800 MHz range, and transmitting 85 watts of 900 MHz right next to the phone desensitized the receiver somewhat. I did manage to get working in enough fits and starts to check the signal strength and start and stop the recording, thank goodness.

The receive antenna was a 7 element (10 dBi) Yagi - about $30 off the Internet, and the reception was almost perfect. That pretty much proves that this transmitter is going to work. With a real antenna - like the 18 dBi loop Yagi I have on order - reception should be possible from perhaps 30 miles away with a good line of sight.

Unlike the first time, it was not necessary to adjust the power. The exciter was set to power level "3", which is an average power of about 0 dBm, or a PEP of about 6 dBm. That means an output power of about 16 watts or so, for an ERP of 80 watts. Of course, all of that is in theory. Still on my to-do list is to run this setup through a proper sampling wattmeter and dummy load to get a measured output power level and get a good measurement on how far down the out-of-channel emissions are. Part 73 contains an emissions mask requirement that says that the out-of-channel emissions must be reduced from the channel average power by at least 47 dB - that is, 36 dB below the pilot. That's a pretty reasonable goal to shoot for. At the power we're contemplating, it'd be less than a couple milliwatts. At 900 MHz, that ought not to be a problem.