Greeting earthlings. If you follow the channel, you know that we love all things Apollo, and
that during our last visit to Steve Jurvetson’s amazing space collection, we were given the
opportunity to take two holy boxes of Apollo electronics to our lab.
These are the boxes that brought you voice, data and live TV from the moon, and should
be early masterpieces of microwave electronics, the blackest of black arts in analog electronics.
In the last episode, we tried to fire up a Motorola PM receiver used by NASA to test
our transponder. We eventually managed to make it work, but at another frequency. We
had just discovered that it had been modified from its Apollo days. It had been changed
to the frequency of another later spacecraft. And it won’t work with our Apollo gear until
we switch it back to its original frequency.
Fortunately, we quickly found out where the quartz crystal that drove the local oscillator
frequency was hiding, under its little golden hood. It said 22 MHz and something on it,
and was marked down to the Hertz, implying an unusually high degree of manufacturing
precision, of less than 1 ppm.
Would it be as simple as changing this crystal to get back our original Apollo frequency?
And how does a 22 MHz crystal control the reception at 2.1 GHz. To find out, we first
need to figure out how the receiver works. We don’t have any documentation for it,
but fortunately we can look at the writing on the modules and get some good clues.
So what do we have in the box, as far as modules are concerned? We already went over the inputs
and the mixer. We've already taken the VCO out to realign it, I’ll show it to you in
a minute. But in terms of modules, it’s quite telling. There is a 50 Megahertz or
Megacycle mixer, and a 10 MHz AGC amplifier. A 10 MHz distribution amplifier, ak 10 Mhz
IF amplifier, a 10 MHz loop phase detector and loop filter. A 20 MHz oscillator and times
3. A X and 1/2 - so that’s a divider by 2, and a 10 MHz reference distribution amplifier.
A CAD AGC amplifier. A 10 MHz wideband phase detector and a 10 MHz narrowband discriminator.
And a video distribution amplifier.
And last but not least, the VCO is already on the bench. So you can see it as three parts.
It’s written 23 Megahertz VCO and x96. But it goes first into this thing. and then into
this thing. And what I think they are, is that part of the multiplayer is here and another
part is there.
And you can kind of tell how they split it up, because on this, this is a passband filter,
133 MHz to 147 MHz, and that's most likely to remove the undesirable harmonics at one
stage of the multiplier, and then it comes up out of this one. And here is our quartz
that’s already taken out. So, out of all that, we should be able to figure out how
this thing works.
So we now have the electronic equivalent of a jigsaw puzzle. And when you put all the
modules together so they fit into each other, you get this, which is how the receiver works.
And what it is, it's a double conversion receiver. Starts in the microwave regime, then gets
down-converted to 50 MHz. And then there's the second conversion to 10 MHz. And in order
to do that, it uses two oscillators. And actually there are two crystal oscillators in this
thing, unlike the transponder. The first one is the one we found in the last episode, it’s
at 23 MHz, gets multiplied by 96. But now we know that it gets multiplied by 6, goes
into the 133-147 MHz filter, and then is multiplied by 16, but it’s the same result. The difference
between those two frequencies is 50 MHz. So here we go, our first conversion. Then the
second stage has its own crystal oscillator. And this time, it's not a VCO that you can
tune, it’s a fixed frequency 20 MHz crystal. Our times 3 multiplier goes here, gives us
60 MHz. 60 minus 50 gives us 10 MHz. And then the divide by 2 gets in the other branch,
so that the 10 megahertz of the down-conversion is compared to the 10 MHz that you obtained
by dividing the crystal 20 megahertz by 2. And that's where you have the phase detector.
And the phase locked loop actually compasses the two oscillators. So it corrects the two
in one fell swoop, but acts only on the frequency on the first one.
So we were right, in order to change the frequency of the receiver, all we have to do is to change
the first conversion, the second one is fixed. So to calculate the new crystal we need,
it's fairly easy.
We want to receive at 2287.5 MHz, and we want an intermediate frequency of 50 MHz. So we
subtract that from the carrier. That should be our local oscillator. And since it's multiplied
by 96 from the crystal, we just divide it 96. This is the crystal frequency that we
should get. 23.30729 MHz. And we need to be very close, because we need
to be within a few 10’s of Hz of the real frequency. But we don't know if it's a parallel
resonant or a serial resonant crystal. And if it's parallel resonant we don't know which
capacitive loading it needs.
So we have encountered crystals previously on the channel, some broken ones actually.
And you know that there's more to a crystal than just its oscillation frequency. In quick
summary, a crystal has two main resonances: the series resonance, and the parallel resonance.
In this VNA trace from the episode on our glitchy Soyuz clock oscillator, the series
resonance is the peak at the left, and the parallel resonance is the inverted peak to
the right. You need to tell the manufacturer which of
the two you want, and if it's a parallel resonance, you also need to specify the capacitive loading
the manufacturer should use to measure the frequency
And of course, it’s not just a matter of changing the crystal, and hope that the circuit
is simply going to oscillate at the new frequency all by itself. We’ll have to retune the
VCO and the multipliers with the many adjustment screws you see poking through the module.
So this looked quite challenging, and we were about to reverse-engineer the whole VCO, when
we went on a little trip to a collector. We had actually to fly there. And it's a collector
that is both generous and very private, he wants to remain anonymous. But guess what
he gave us?
I snapped this in the plane on our way back. It says PM Receiver Support Manual and it’s
from Motorola. Hmm, does this box look familiar? Yes, that’s our receiver, and that is its
missing manual. And of course, we went straight to the parts section, and there at the bottom
was our crystal! Good news, we had calculated our crystal frequency correctly. And then
of course, we frantically searched if there was a section on the tuning of the VCO. And
guess what, there is! The whole procedure in glorious details! No need to reverse engineer
the VCO anymore!
And it also says that the crystal controlled VCO is in a series resonant loop, so it's
probably a series resonant crystal.
And here is the oscillator in question. And you see the quartz crystal, the varicaps,
so it definitely looks like a serial thing. But it’s loaded with caps and Ls, and I
thought you could only tune the parallel resonant frequency. So right now we have a quartz at
a different frequency in it, we extracted it. And I want to figure out if it’s indeed
oscillating at the serial resonant, or the parallel resonant frequency, so I can order
the right quartz.
So that’s the crystal in question that's in there. So here I have my little tester
that I used in the previous video, and I'm already all calibrated.
You see the series resonance and the parallel resonance, and I was expecting this to be
a series resonant crystal. But, no! The series resonance is right here, 22.8842,
which is not 22.885417! That's completely off. And of course the parallel is over here,
which is 22.881. But to me, that tells me that this is not a serial crystal, that’s
a parallel resonance crystal. Let’s put it where it should oscillate. And I am on
the atomic source for this, so it’s very precise.
So here is my unloaded crystal that resonates at none of the advertised frequency that’s
on its face. And then if I put a 15 pF cap, now it’s smack in the middle. It’s exactly
where it should be.
Okay we wanted to do a simple experiment to settle for good if the crystal should be cut
for serial or parallel resonance. OK, what I am just going to do is power the thing on
and then tune it manually, and we’re going to record the range where it goes.
It moves 1kHz. So we really have to order the quartz absolutely perfect, we don’t
have much leeway at all. This is in the middle of the range, I get 22.885415, and the quartz
is 22.885417. It is oscillating in the middle of its parallel resonance. Which makes sense,
right, because the parallel resonance is the one you can tune. So we have to order a crystal
on that frequency, parallel resonance, 15 pF loading. OK, phew!
Wait, there is more to this story, and the crystal is back on the VNA. Because we found
a specialty crystal manufacturer, and we confidently told him, we knew exactly what we wanted:
a crystal in that case, parallel resonance, 15 pF loading, the exact frequency. And then
he came back and said: well it's weird, the crystal is in such a big case. Are you sure
it's not a crystal that’s designed to work on a harmonic? So his point was that if it
were a 23 MHz crystal working on the fundamental, the crystal would be so thin that you wouldn't
put it in a case like this. So, he thinks it's a third overtone crystal. And I believe
he's right. And then, if we go and broaden our point of view… How do I do that? It
says recall, and I have one that goes from 4 to 30 MHz. Recall. And now I have completely
changed the scale. Now it starts at 4 MHz and goes to 30 MHz. And sure enough, there's
a big resonance around 7 MHz, and that is the fundamental. So this guy is working on
its third overtone. And I asked the guy if it made a difference if it were in a VCO,
if you were to tune the cyrstal. He said yes, it will to at a completely different rate,
if you work on the 3rd overtone. So good catch Mr. Crystal Manufacturer. I’ll respond to
him that yes, he was right, and that we need to add to the specification that we want the
third overtone crystal.
So our crystal is on order, and it is being lovingly polished by our Canadian friends
right now, by a company called Laptech Precision, and yes, they really know about their crystals.
All right, we now just have to wait for the crystal. In the meantime, we have plenty to
keep us busy, as one additional test equipment piece got sent by our generous collector.
We finally have the PM transmitter, the one important piece we that was missing from our
test setup. If it keeps going like that, we may soon have to power up complete spaceship.
Anyhow, see you in the next episode!