Hi Pete,

At 10mA, the stock RF bandswitching diodes in the DX-394 for the SW bands is
spec'd to have a resistance of about 0.2 ohms, or 1/20 that of the 1N5767
PIN diode, and a capacitance of about 0.6pF with a reverse bias of 13V. The
Hitachi HSU277 is described as a Silicon Epitaxial Planar Diode for UHF/VHF
tuner Band Switch. I cannot tell from the spec sheets why a PIN diode would
be superior in this application. What should I be looking for? Do you think
there is much to be gained by replacing them?

BTW, the LW/MW bandswitching diodes are Toshiba 1SS272 Silicon Epitaxial
Planar with no spec for the resistance and a capacitance of about 0.8 pF.

73, Tom

P.S. - sorry, I first sent this privately by accident. We'll assume others
are interested!

"Pete KE9OA" wrote in message
...


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.

This amount of current would be fine..........I think that the resistance
would be about 5 ohms with this bias level.



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.

I am not sure about that one.



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.

You should be ok...........the main thing that would be affected would be
the close-in IP3.



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

Tom, those specs are fine...........the thing that catches my attention is
the relatively low impedance of the crystal filter. Typically, these
filters have an IN/OUT Z of between 1500 and 7000 ohms. I would surmise
that the current filter has around a 3000 ohm impedance. Take a look at
any series/shunt resistors around the filter area and it might give a
clue.
A series L, shunt C would do the trick if you need to do any impedance
matching.
I have created a dual-purpose spreadsheet that does these calculations; a
second page of this spreadsheet does PLL loop filter calculations that are
based on Tom Wheeler's equation set. This is for the double-ended phase
detector output circuit that uses the Phi V and Phi R outputs.

Pete