Multiplier chains
 

Hello all,
I am trying to muliply a 72MHz crystal oscillator to 1296 MHz. My
first approach was to just make a series of X2 or X3 transistor
mulipliers to get the require X18 multiplication fact. However, I have
found two pieces of equipment in the junk box which just use diode
multipliers. One is an old Electrophone UHF CB radio which multiplies
the crystal oscillator by 17 (I think, from memory)then uses helical
resonators to filter the desired harmonic. The same approach is used in
an old King aircraft transponder where the 138 MHz crystal is multiplied
to 960 MHz with just a diode and uses the first two stages of an
interdigital mixer to get rid of unwanted harmonics.
So, the question is, which is the better approach? I just want a
reasonably clean signal source to test a 1296 MHz down-converter.
The diode approach seems simpler but is it likely to contain more
spurious signals than a transistor multiplier chain?
Alan
VK2ADB

Multiplier chains
 

On Wed, 21 May 2008, Alan Peake wrote:

Hello all,
I am trying to muliply a 72MHz crystal oscillator to 1296 MHz. My first
approach was to just make a series of X2 or X3 transistor mulipliers to get
the require X18 multiplication fact. However, I have found two pieces of
equipment in the junk box which just use diode multipliers. One is an old
Electrophone UHF CB radio which multiplies the crystal oscillator by 17 (I
think, from memory)then uses helical resonators to filter the desired
harmonic. The same approach is used in an old King aircraft transponder where
the 138 MHz crystal is multiplied to 960 MHz with just a diode and uses the
first two stages of an interdigital mixer to get rid of unwanted harmonics.
So, the question is, which is the better approach? I just want a reasonably
clean signal source to test a 1296 MHz down-converter.
The diode approach seems simpler but is it likely to contain more spurious
signals than a transistor multiplier chain?
Alan
VK2ADB


I don't know, and one thing to remember is that what was done years ago
may no longer be the solution because other things have come along.

A single stage of multiplication is of course simplest. But, if you
do it in one step, the signal may be so weak that you need stages of
amplification at the ultimate frequency. Once upon a time, frequency
limits may have made that unfeasible.

Also, if you have one stage that basically generates harmonics, and
then you expect to pick off the desired frequency, that filtering
may need to be much better than multiple stages. If you start
with a low enough crystal frequency, the next harmonic may be too close
and some of it will get through the filter on the ultimate frequency.
If you have a string of multipliers, each does filtering so the next
stage only has to deal with filtering out a relatively high frequency.

Note that your two examples aren't comparable. The first example you
say multiplies by 17, while the second only multiplies by 6. Even in
the old days, it wasn't uncommon to see a jump like 6, but something
like 17 was less common.

The real trick seems to be to start with as high a frequency as possible.
Then the multiplication needed is limited, and it's far easier to filter
out harmonics from a higher frequency crystal than a lower one.

Michael VE2BVW

Multiplier chains
 

On Wed, 21 May 2008 17:00:40 +1000, Alan Peake wrote:
Hello all,
I am trying to muliply a 72MHz crystal oscillator to 1296 MHz. My
first approach was to just make a series of X2 or X3 transistor
mulipliers to get the require X18 multiplication fact. However, I have
found two pieces of equipment in the junk box which just use diode
multipliers. One is an old Electrophone UHF CB radio which multiplies
the crystal oscillator by 17 (I think, from memory)then uses helical
resonators to filter the desired harmonic. The same approach is used in
an old King aircraft transponder where the 138 MHz crystal is multiplied
to 960 MHz with just a diode and uses the first two stages of an
interdigital mixer to get rid of unwanted harmonics.

So, the question is, which is the better approach?

Are you building tens of thousands like Electrophone and King did?
There was more than 'good' engineering involved with those boxes.
The bean counters were all over it, too -- I'm sure.

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
*** Killfiling google posts: http://jonz.net/ng.htm

Multiplier chains
 

On Wed, 21 May 2008 17:00:40 +1000, Alan Peake
wrote:

However, I have
found two pieces of equipment in the junk box which just use diode
multipliers. One is an old Electrophone UHF CB radio which multiplies
the crystal oscillator by 17 (I think, from memory)then uses helical
resonators to filter the desired harmonic.


Sounds like a step recovery diode (SRD) multiplier, which generates a
very broad spectrum. In extreme multiplication situations a tunable
harmonic analyser could be used to select out a single harmonic from
the broad SRD spectrum (mix down with a VFO, filter with a crystal
filter with bandwidth comparable to the fundamental and mix up the
filtered harmonic up to the original frequency using the _same_ VFO,
effectively canceling out any VFO drift).

However, since there have been prescalers available for quite a while
for satellite-TV indoor units, which are capable of at least 2 GHz,
why not use a PLL. Put a VCO at 1296 MHz, use a divide it by 64 using
the prescaler and phase lock it to a 20.25 MHz crystal. With such high
reference frequency, the loop filter can be quite broad, greatly
attenuating the VCO phase noise.

Paul OH3LWR

Multiplier chains
 

You could consider another approach -- phase locking a 1296 MHz
oscillator to your 72 MHz xtal.

Dr. G.


Alan Peake wrote in news:4833C898.2090302
@nosspam.activ8.net.au:

Hello all,
I am trying to muliply a 72MHz crystal oscillator to 1296 MHz. My
first approach was to just make a series of X2 or X3 transistor
mulipliers to get the require X18 multiplication fact. However, I have
found two pieces of equipment in the junk box which just use diode
multipliers. One is an old Electrophone UHF CB radio which multiplies
the crystal oscillator by 17 (I think, from memory)then uses helical
resonators to filter the desired harmonic. The same approach is used in
an old King aircraft transponder where the 138 MHz crystal is
multiplied
to 960 MHz with just a diode and uses the first two stages of an
interdigital mixer to get rid of unwanted harmonics.
So, the question is, which is the better approach? I just want a
reasonably clean signal source to test a 1296 MHz down-converter.
The diode approach seems simpler but is it likely to contain more
spurious signals than a transistor multiplier chain?
Alan
VK2ADB

Multiplier chains
 

Is there some magic to using the 72 MHz oscillator? Gigahertz synthesizers
are cheap and simple these days. Back in the bad old days multiplication
was the cheap and easy way to get stable high frequencies. Today, that
helical resonator would probably buy the synthesizer chip and associated
support circuitry, and you needn't fiddle around with stage after stage.

...

"Alan Peake" wrote in message
...
Hello all,
I am trying to muliply a 72MHz crystal oscillator to 1296 MHz. My
first approach was to just make a series of X2 or X3 transistor mulipliers
to get the require X18 multiplication fact. However, I have found two
pieces of equipment in the junk box which just use diode multipliers. One
is an old Electrophone UHF CB radio which multiplies the crystal
oscillator by 17 (I think, from memory)then uses helical resonators to
filter the desired harmonic. The same approach is used in an old King
aircraft transponder where the 138 MHz crystal is multiplied to 960 MHz
with just a diode and uses the first two stages of an interdigital mixer
to get rid of unwanted harmonics.
So, the question is, which is the better approach? I just want a
reasonably clean signal source to test a 1296 MHz down-converter.
The diode approach seems simpler but is it likely to contain more spurious
signals than a transistor multiplier chain?
Alan
VK2ADB

Multiplier chains
 

xpyttl wrote:
Is there some magic to using the 72 MHz oscillator? Gigahertz synthesizers
are cheap and simple these days. Back in the bad old days multiplication
was the cheap and easy way to get stable high frequencies. Today, that
helical resonator would probably buy the synthesizer chip and associated
support circuitry, and you needn't fiddle around with stage after stage.

I just wanted to do it with existing bits.
Alan

Multiplier chains
 

Michael Black wrote:

I don't know, and one thing to remember is that what was done years ago
may no longer be the solution because other things have come along.

Yes, as others have pointed out, a VCO and PLL is a better solution.

A single stage of multiplication is of course simplest. But, if you
do it in one step, the signal may be so weak that you need stages of
amplification at the ultimate frequency. Once upon a time, frequency
limits may have made that unfeasible.
I have a supply of MAR-1s that should do the trick there.


Also, if you have one stage that basically generates harmonics, and
then you expect to pick off the desired frequency, that filtering
may need to be much better than multiple stages. If you start
with a low enough crystal frequency, the next harmonic may be too close
and some of it will get through the filter on the ultimate frequency.
If you have a string of multipliers, each does filtering so the next
stage only has to deal with filtering out a relatively high frequency.

I wouldn't try a diode to go from 72 to 1296 but will double or triple
the oscillator and perhaps try the diode from there.

Note that your two examples aren't comparable. The first example you
say multiplies by 17, while the second only multiplies by 6. Even in
the old days, it wasn't uncommon to see a jump like 6, but something
like 17 was less common.
Sorry, it was a factor of 9, not 17. I just found the circuit:) The xtal
was 17 MHz.


The real trick seems to be to start with as high a frequency as possible.
Then the multiplication needed is limited, and it's far easier to filter
out harmonics from a higher frequency crystal than a lower one.



Michael VE2BVW
Thanks for your thoughts.
Alan

Multiplier chains
 

Allodoxaphobia wrote:

Are you building tens of thousands like Electrophone and King did?
There was more than 'good' engineering involved with those boxes.
The bean counters were all over it, too -- I'm sure.

Jonesy

No, this is a "one-off" from my junk box.
Alan

Multiplier chains
 

Paul Keinanen wrote:

Sounds like a step recovery diode (SRD) multiplier, which generates a
very broad spectrum.
......
I have a couple of SRDs but they seem to be optimised for 500MHz input
and I don't know if they would work at lower frequencies. I would assume
that the diodes I mentioned are some sort of SRD or "snap" diode so I
might pull one out and try it.

Paul OH3LWR