An assortment of built and tested boards built "ugly method" for a 17m SSB/CW
superhet txvr (20 MHz IF) and a 6m CW DSB/CW txvr. In the lower right corner is
a 20MHz xtal osc driving a buffer driving a diode doubler driving a 2 pole band
pass filter to get 40 MHz to mix with a 10.1 MHz Hartley VFO to get a 50.1 MHz
LO source for the 6m DSB txvr. After I get all the bugs worked out I find that the
local surplus store has 40 MHz TTL osc cans for 50 cents each! And they will put
out over 16dBm for 5v@20mA. I am now a fan of osc cans. The catalogs have them for
about $2 new. So the plan now is to go with a can and resistive split the +16dBm
in to a pair of +7dBm LOs--one for the DSB balanced modulator and one for the direct
conversion rx diode ring product detector.
UPDATE Feb 2006--- Yes the osc cans have some merits. However, further measurements
have shown that the phase noise of the cans is really bad! I have since abandoned them,
other than maybe for band edge markers and the like.
A shot of the 20 MHz xtal filters built and tested.
My Cohn topology ladder filter experimentation led me to prefer the idea of making
filters in sets of four xtals each. Then end loading for passband ripple can be
controlled easily. Then cascading a pair of "fours" with a buffer amp and pad between
them for 8 resonators total became my goal.
I found that 5 xtals seems to be the limit for retaining reasonable pass band shape
and ripple control with external end loading. Conversations with Rick Campbell, KK7B,
verified this observation.
More than 5 xtals isolates the inner xtals from the effects of end loading and you get
stuck with pass band shapes and ripple that you have little control over.
These filters are intended to be the heart of a 17m superhet SSB/CW txvr. It turns out
that a 20 MHz IF and a 1.9 MHz LO make for a marginal mixing combination for 18.1 MHz.
Further measurements indicate that the low side LO will probably not work for this band/IF combination.
Mixing spurs/products are not easily removed from the 18.1 MHz preferred signal.
To the left is a 17m low pass filter. I may opt for a 7th order .1dB Chebycheff instead.
On the right is a 2 pole Butterworth band pass filter for 18.1 MHz.
The center coupling cap is an air variable that was used since I could not find a
convenient value out of my minimalist selection at the time I built it.
The 17m rx first mixer and post amp are in the center.
As of early 2005, I have now essentially abandoned the Cohn topology (equal coupling
coefficients between resonators) in favor of real design. With the advent of GPLA
(General Purpose Ladder Analysis from W7ZOI), along with the work of Bill Carver, W7AAZ
and Jacob Makhinson, N6NWP, it is hard to justify not doing it right by designing a
real filter that can actually produce a legitimate Chebychev response (SSB) with 6
or more crystals and well behaved skirts, along with well behaved and controlable
pass band ripple.
If I had the task of getting a filter up and running in an hour, then I would want
to brute force it with the Cohn topology (partially emperical choice of coupling cap values).
However, a few hours of crystal characterization (extracting freq, motional L,
motional C, Qu) and simulation are well worth it to get 10 xtals with .1dB Chebychev
repsonse with only 2.5dB of insertion loss and good control of bandwidth that is essentially
independent of response characteristics (to a first order).
Or a real design for a proper time domain response ladder filter for CW purposes.
My all band HF transceiver. This is a superhet rx with 9 MHz IF.
I use a 5-5.5 MHz Hartley VFO reference in an analog one-on-one phase locked loop
for high side LO injection for each band.
The RX front end is a BPF/LNA in to a level 17 diode ring with broad band NPN post amp.
A post LNA BPF is still needed here.
This rig uses IF derived AGC with a pair of cascaded MC1350s.
The TX chain is a diode ring DSB modulator with some gain feeding into the 9MHz
IF xtal filter. Broad band gain stages then feed in to a diode ring transmit mixer,
followed by a double tuned band pass filter driving more broad band feed back amps
in to a balanced pair of IRF511 HEXFETs.
All TX low pass filters are 7th order .1dB Chebycheff.
The low pass filters are relay switched.
The band pass filters are diode switched. This rig is still missing a couple
of crystals and needs some more work on the finals to be complete. Output is maybe
8 watts on 75m at Vcc=13.8v.
My 20 MHz xtal filters for a 17m SSB/CW transceiver.
I opted for 4 xtals, broad band amp and then 4 xtals.
They were surplus xtals and I found that a low side LO works
for 1.9 MHz LO and 20 MHz IF for receive but not for transmit.
I went with the low freq LO so that drift would be easier to manage.
It turned out that a low drift LO at 1.9 MHz was more of a nuisance than
I had thought it would be.