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Author Topic: DSB design problems  (Read 4043 times)
N7ORL
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Posts: 5




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« on: April 19, 2001, 10:11:59 AM »

I am currently on my fifth attempt to build a workable
75 meter dsb transmitter (3-5 watts)  I have tried just about every scheme.  From the handbook I tried the mc1496 mixer setup... really frustrating...  Diode mixers,
some success, but broadband crud and poor audio made them unacceptable to me.  Has anyone here actually built a reliable, repeatable circuit that will give a couple of watts on 75??  Thanks,  73, Mike.
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WB6BYU
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Posts: 13287




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« Reply #1 on: April 19, 2001, 11:43:29 AM »

try http://www.alphalink.com.au/~parkerp/proj80ds.htm

I'd also recommend looking through Solid State Design for the
Radio Amateur by DeMaw and Hayward.  There are several
parameters which need attention:  the correct VFO injection to the
mixer, a clean audio amp, impedance matching between stages,
and linear RF stages following the mixer (not class C.)

Good luck!  (And I hope you will share the circuit when you get it
going.)  - Dale WB6BYU
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N7ORL
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Posts: 5




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« Reply #2 on: April 19, 2001, 09:27:02 PM »

Tnx fer reply.  Yup, I have checked all of the parameters you've mentioned (I think!!)  Anyway, obviously missing something there on every trial.
Will give that site a try.  I think I may have already
but worth a shot.  --...  ...--  Mike.
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WB6BYU
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Posts: 13287




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« Reply #3 on: April 20, 2001, 12:18:10 PM »

It is difficult to guess what is going wrong without evaluating each
stage.  Bad audio sounds like distortion or clipping somewhere -
impedance matching, incorrect biasing, or an intermittent in the
mic cable.  I had this problem in one rig and traced it to inadequate
VFO injection to the mixer (but could be improper audio drive to the
mixer as well.)  Wideband garbage on the output could be due to a
low-frequency oscillation in one of the stages.  Did you use an
audio filter before the mixer?  Since there is no crystal filter, using
a mic with a wide audio response up to 15 kHz could create a
30 kHz wide signal.  A low-pass filter set for 3 kHz (or a bit higher,
depending on the desired fidelity) will reduce the bandwidth of the
transmitted signal.

(And I presume you've checked for RF feedback into the mic amp?)

For debugging, the best approach is to make sure each stage is
working properly - listen to the output of the audio amp, listen to
the output of the balanced modulator on a separate receiver to
make sure the signal out from it is clean, drive the amplifier chain
from another source to make sure it is not adding distortion, etc.

As for appropriate circuits:  any good SSB transmitter circuit can be
used as the basis of a DSB transmitter - just change the oscillator
frequency to generate the signal at the output frequency and
remove the crystal filter following mixer stage.

Perhaps you can share more about what you have tried and your
measurements, and someone might have more ideas of what to do
differently.

Good luck ! - Dale WB6BYU
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N7ORL
Member

Posts: 5




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« Reply #4 on: April 21, 2001, 06:43:32 AM »

I picked up W1FB's book  on QRP... Excellent book!!
Although he uses no less than five stages to get 1/2 watt of DSB signal!! It is very close to the design
I had the "best" success with.  Checking my actual L.O.
injection level is where I may have gone awry in the past. I assumed too much I think.  I believe the wideband crud
was caused by induced parasitic oscillation, possibly peculiar to the layout I'd chosen. Thanks for the reading suggestions, they were helpful.  Just curious Dale,
Which DSB setup have you had the best success with??
DeMaw has a phasing SSB exciter in there that is tempting to try, though he isn't specific on many components.   Tnx and 73  Mike.
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VK3YE
Member

Posts: 16




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« Reply #5 on: April 22, 2001, 06:14:52 AM »

I've found that the NE602 is the easiest balanced
modulator chip around.  A 741 or similar will
work fine as a microphone amp.  You will need
about three transistors to get to 2w from the output
of an NE602.  An IRF510 is a good PA.
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VK3YE
Member

Posts: 16




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« Reply #6 on: April 22, 2001, 06:15:46 AM »

I've found that the NE602 is the easiest balanced
modulator chip around.  A 741 or similar will
work fine as a microphone amp.  You will need
about three transistors to get to 2w from the output
of an NE602.  An IRF510 is a good PA.
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WB6BYU
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Posts: 13287




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« Reply #7 on: April 23, 2001, 01:03:39 PM »

I'm sure VK3YE has more experience at DSB projects than I have!
My problem was actually with an SSB rig, when I replaced the
RF board on my Ten-Tec Argosy with one from the newer digital
version.  The transmit mixer had changed from an LM1496 to a
diode ring, and the existing VFO didn't like the lower impedance
it had to drive.  I got several complaints of "bad audio", but it took
me several years to get around to tracking down the problem since
I work mostly CW.  Turns out the VFO output was saturating due to
the heavy loading.  I had a great idea for an AGC-controlled RF
amp to maintain constant injection level on the mixer, but settled
for a simple 4 : 1 transformer to provide a better impedance match
between the stages.

One of the reasons why you see several stages of amplification
following the DSB generator is because they have to be linear, and
will typically have less gain than the class-C stages usually used
in CW transmitters.  The low-level stages will probably be class-A,
which means significant current draw even when there is no signal.
Overdriving, clipping, or any other non-linearities in the amplifier
stages after the modulator will generate a wide, "garbage" signal.
If you are "borrowing" the RF stages from another design, make sure
they are designed for SSB instead of CW.  (But SSB rigs typically
have some IF amplification in addition to the RF stages.)
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N7ORL
Member

Posts: 5




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« Reply #8 on: April 23, 2001, 08:17:32 PM »

I had checked the biasing, as this was my first guess.
All of the stages should have been in the linear region with .7 to 1 volt forward on the bases.  All were drawing appreciable standby current naturally.
What I wouldn't give for a spectrum analyzer.  Anyway
I haven't given up yet... may try Peter's suggestions
The IRF510 looks tempting as a final. The NE602 should make a good mixer. I have a homebrew DC receiver that uses one, and it works beautifully.  73  Mike.
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