Has anybody ever changed the 1st mixer diodes to hot-carrier types. I have
one of these in my possession and the IMD is terrible. They've got to be
using ordinary switching diodes for the performance to be this bad. I notice
that they are using a 2N5109 post mixer amplifier, so I don't understand why
they would skimp on the mixer diodes.

Pete

With the new mixer, all of the IMD products are gone. I still need to match
the mixer output to the 30MHz crystal filter, but it looks like I am on the
right track.............

Pete

"Pete KE9OA" wrote in message
...
Has anybody ever changed the 1st mixer diodes to hot-carrier types. I have
one of these in my possession and the IMD is terrible. They've got to be
using ordinary switching diodes for the performance to be this bad. I
notice that they are using a 2N5109 post mixer amplifier, so I don't
understand why they would skimp on the mixer diodes.

Pete

Hi Pete,

I usually learn something useful from your postings and enjoy reading about
the repairs and mods you've made and the progress on the MW receiver. I've
got some questions you might be able to help me with regarding the DX-394
and intermod. I'm wondering what one might do to improve its strong signal
intermod performance.

It uses HSU277 diodes to switch among 3 SW preselector bands and 1SS272
diodes for the LW/MW bands. I have nearly doubled the reverse bias and
forward current by switching from the 7V supply to the 13.8V supply with
little obvious benefit. At best, I can shove 10-12 mA through them. Is there
any point in changing the diodes? If so, what would you suggest? I think
SMD's are preferable so as not to degrade stray coupling.

The RF preamp is a 3SK195 and the 1st mixer uses a pair of these balanced
for RF and driven in parallel by the LO. Is there any point in paralleling a
second 3SK195 (piggyback style) on the preamp? on the mixer? Or changing out
the transistor type? How would one determine/set the correct operating
point?

The 1st IF (45 MHz) filter is a 2-pole crystal filter with 15kHz bw at -3dB,
100kHz at -24 dB. I acquired a 30kHz/-3dB, 120kHz/-40dB matched pair that I
was thinking of substituting in order to widen bandwidth for DRM. Note that
the centre of the 2nd IF tunes across a 5kHz segment of the 1st IF passband
so that a 10kHz or wider bw at the 2nd IF rolls off on one side or the other
because of the shoulders of the 15kHz 1st IF. Is this a bad idea for
intermod? The stopband attenuation is going to be poorer out to maybe 50-60
kHz bw but should be better beyond that, apart from stray coupling due to
squeezing in a pair of filters where one would ordinarily be.

What are your thoughts?

73, Tom

I usually learn something useful from your postings and enjoy reading
about the repairs and mods you've made and the progress on the MW
receiver. I've got some questions you might be able to help me with
regarding the DX-394 and intermod. I'm wondering what one might do to
improve its strong signal intermod performance.

It uses HSU277 diodes to switch among 3 SW preselector bands and 1SS272
diodes for the LW/MW bands. I have nearly doubled the reverse bias and
forward current by switching from the 7V supply to the 13.8V supply with
little obvious benefit. At best, I can shove 10-12 mA through them. Is
there any point in changing the diodes? If so, what would you suggest? I
think SMD's are preferable so as not to degrade stray coupling.

Did you do the I.F mods that are on www.mods.dk ? I remember that this
receiver didn't have the I.F. filters terminated properly and the result was
signal feedaround, which manifested itself as being able to hear adjacent
signals superimposed on the desired signal. An example of this would be
listening to KAAY on 1090 and hearing that Cleveland station from 1100kHz
coming in as a background signal. Not splatter, mind you, but sounding as if
it is on 1090 itself.
Now, as far as filter switching diodes, my favorite (doesn't mean that this
is the best) is the 1N5767 PIN diode, biased at around 75mA. I have had good
results using leaded devices.

The RF preamp is a 3SK195 and the 1st mixer uses a pair of these balanced
for RF and driven in parallel by the LO. Is there any point in paralleling
a second 3SK195 (piggyback style) on the preamp? on the mixer? Or changing
out the transistor type? How would one determine/set the correct operating
point?

The radio doesn't seem bad in this respect. I haven't had one for several
years, so I wouldn't be able to give an intelligent answer without the
schematic. You could look at the Icom R75 schematic and see how they
implement their RF amp. I believe that they do use the configuration that
you suggest.

The 1st IF (45 MHz) filter is a 2-pole crystal filter with 15kHz bw
at -3dB, 100kHz at -24 dB. I acquired a 30kHz/-3dB, 120kHz/-40dB matched
pair that I was thinking of substituting in order to widen bandwidth for
DRM. Note that the centre of the 2nd IF tunes across a 5kHz segment of the
1st IF passband so that a 10kHz or wider bw at the 2nd IF rolls off on one
side or the other because of the shoulders of the 15kHz 1st IF. Is this a
bad idea for intermod? The stopband attenuation is going to be poorer out
to maybe 50-60 kHz bw but should be better beyond that, apart from stray
coupling due to squeezing in a pair of filters where one would ordinarily
be.

What are your thoughts?

73, Tom

Tom, you should be able to use a wider bandwidth crystal filter. The natural
impedance of the filter would probably be different than the current filter.
If the impedance matching is not correct, you will have a couple of
problems.
First of all, the insertion loss and passband ripple can increase. If group
delay is important for DRM, this could be a problem. I don't know if bit
error rate is a consideration in this case.
Secondly, I would take a look at the 910kHz rejection of the filter that you
are considering. If it is not high enough, you will be able to hear that (2
X 2nd I.F) response.

"Pete KE9OA" wrote in message
...


Did you do the I.F mods that are on www.mods.dk ? I remember that this

Thanks for the response, Pete. Yes, very effective. BTW, I posted the mods
there.

Now, as far as filter switching diodes, my favorite (doesn't mean that
this is the best) is the 1N5767 PIN diode, biased at around 75mA. I have
had good results using leaded devices.

Is that still a good choice if I can push only 10-12mA? It would require a
complete redesign to go higher.


The RF preamp is a 3SK195 and the 1st mixer uses a pair of these balanced
for RF and driven in parallel by the LO. Is there any point in
paralleling a second 3SK195 (piggyback style) on the preamp? on the
mixer? Or changing out the transistor type? How would one determine/set
the correct operating point?

The radio doesn't seem bad in this respect. I haven't had one for several
years, so I wouldn't be able to give an intelligent answer without the
schematic. You could look at the Icom R75 schematic and see how they
implement their RF amp. I believe that they do use the configuration that
you suggest.

I really meant piggyback! No additional components but probably would have
to change out the resistors to set the operating point correctly. Wondered
if it was a crazy idea or had merit.


The 1st IF (45 MHz) filter is a 2-pole crystal filter with 15kHz bw
at -3dB, 100kHz at -24 dB. I acquired a 30kHz/-3dB, 120kHz/-40dB matched
pair that I was thinking of substituting in order to widen bandwidth for
DRM. Note that the centre of the 2nd IF tunes across a 5kHz segment of
the 1st IF passband so that a 10kHz or wider bw at the 2nd IF rolls off
on one side or the other because of the shoulders of the 15kHz 1st IF. Is
this a bad idea for intermod? The stopband attenuation is going to be
poorer out to maybe 50-60 kHz bw but should be better beyond that, apart
from stray coupling due to squeezing in a pair of filters where one would
ordinarily be.

What are your thoughts?

73, Tom

Tom, you should be able to use a wider bandwidth crystal filter. The
natural impedance of the filter would probably be different than the
current filter. If the impedance matching is not correct, you will have a
couple of problems.
First of all, the insertion loss and passband ripple can increase. If
group delay is important for DRM, this could be a problem. I don't know if
bit error rate is a consideration in this case.
Secondly, I would take a look at the 910kHz rejection of the filter that
you are considering. If it is not high enough, you will be able to hear
that (2 X 2nd I.F) response.

I am looking for a wider, flatter response around the uniform spectral
density of the common 10kHz DRM channel for better group delay but I'm not
sure that DRM is so very susceptible because there is pronounced and dynamic
group delay inherent in ionospheric propagation. A 1dB ripple is considered
ideal so I imagine that a flat amplitude-frequency and linear
phase-frequency response in the radio have a small beneficial effect on the
rate of successful decoding. There is also a 20kHz wide DRM mode.

The 4-pole cascaded filter is spec'd to have minimum attenuation of 70dB
at +/- 910kHz and 800ohms/1pF terminating impedance. The filter it replaces
is unspecified at +/-910kHz but I would guess that it is 35-40 dB; its term
impedance is 560/6 ohms/pF. So the 910kHz image suppression should be
improved by up to 35 dB less strays. Filter loss will increase by 1.5 dB per
spec. If the circuit currently matches the stock filter, is the ripple and
increased loss liable to be severe?

73, Tom

In article
,
"Pete KE9OA" wrote:

Has anybody ever changed the 1st mixer diodes to hot-carrier types. I have
one of these in my possession and the IMD is terrible. They've got to be
using ordinary switching diodes for the performance to be this bad. I notice
that they are using a 2N5109 post mixer amplifier, so I don't understand why
they would skimp on the mixer diodes.

I'm have no experience with radio RF mixer circuits but maybe you are
just over driving it?

--
Telamon
Ventura, California

Under the same conditions, none of my other radios don't have this problem.
After substituting the 1st mixer for the MCL unit, the IMD products are now
gone. Another problem is the approach the designers took with this radio.
The gain distribution is not set up properly. Instead of having the RF
amplifier ahead of the 1st mixer, they should have placed the 2N5109 after
this mixer.
I still need to do more work with the impedance matching network that
follows the mixer. I have installed a diplexer, but I really need to measure
the 30MHz crystal filter impedance so I can properly calculate the values
required for a matching network.
A series L, shunt C seems to work pretty well in this application. With the
initial calculations for a 3k impedance, I have reduced the dip in the
response down to 2dB, but I know that I should be able to decrease the
passband ripple to less than 1dB.

Pete

"Telamon" wrote in message
...
In article
,
"Pete KE9OA" wrote:

Has anybody ever changed the 1st mixer diodes to hot-carrier types. I
have
one of these in my possession and the IMD is terrible. They've got to be
using ordinary switching diodes for the performance to be this bad. I
notice
that they are using a 2N5109 post mixer amplifier, so I don't understand
why
they would skimp on the mixer diodes.

I'm have no experience with radio RF mixer circuits but maybe you are
just over driving it?

--
Telamon
Ventura, California