Because both radios are the same design, I suppose it's possible that what I
am observing is as much the fault of RX2 as it is of RX1 except that the
output of RX1 at 455kHz only drives RX2 to about S9 while the test signal
into RX1 drives it much higher. I wish I had a spectrum analyser. I might be
able to confirm that the effect is or is not the influence of the second RX2
by checking the output of RX1 with my boat anchor analog R4B - it cannot
receive 455kHz but I was observing the effect with RX2 tuned to the 10m ham
band, too.

The DX-394 comprises a 3SK195 RF amp, double 3SK195's for the 1st mixer
differentially driven by the r.f. amp and in common mode by the 1st LO,
45MHz crystal filter, 2SC27140 1st IF amp, 2SK210Y 2nd mixer, diode
switches for choice of two filters. The 1st LO PLL is a Fujitsu
MB87014APF-G-BND referenced to a 33MHz crystal all under a soldered shield.
From the schematic, (I'm no expert at reading these) pin 12 (designated as
the output for an active type low pass filter) feeds what looks to be a
control for voltage applied to a varactor diode with a feedback path
consisting of a 33k in parallel with a 0.01 (M - mylar?) cap both in series
with a 0.033 (M) cap. That would be the low pass filter and would control
settling time, right?

73, Tom

Pete KE9OA wrote:
I would have to agree, that it is the reference sidebands
of the LO that you are seeing................it sounds
like they are using a relatively wide loop filter in the
synthesizer, in order to achieve a fast settling time.

Pete

Richard Hosking wrote in message
. au...
Tom
What is the spectrum going into the mixer at the output
of the crystal filter? If this is OK, then it must be
the second mixer that is at fault. Look at the spectrum
of the LO into the first mixer - does it have 5KHz
reference sidebands, and at what level?
What is the 1st mixer and post mixer amp? Are they able
to cope with the strong broadcast signal? If they are
not something substantial like a ring diode mixer and a
strong post mix amp, then the IM might be happening
there. A typical bipolar mixer might only be able to
cope with -20dBm without overload. If the RF gain doesnt
help, then maybe it is the RF amp that is at fault,
though I wouldnt expect all this to get through the
crystal filter. (though even crystal filters can be
nonlinear if driven hard enough...)

Richard

Tom Holden wrote in message
.. .
I was startled when I hooked up a DX-394 radio (RX2) to
the output of the 2nd mixer of another DX-394 (RX1) to
find that the strong MW station RX1 was tuned to showed
up not only at the 2nd IF of 455 kHz but at every 5kHz
from around 200kHz to 30MHz! The strength varied with
frequency, probably in part with the gain vs frequency
curve of RX2. There were stronger signals at the 2nd
and 3rd harmonics of 455kHz. Had to reduce the RX1's RF
gain to the point where its AGC was inactive and the
receiver was operating just above its threshold of
detection to diminish the forest to just a few trees.

The 2nd mixer is preceded by a crystal filter at the
1st IF of 45MHz +/-2.5kHz so I expected to see spectrum
of about 15-20kHz width across 455kHz and some 3rd and
higher order products like 910 and 1365kHz. The 5kHz
interval is suspiciously the same as the steps in the
1st LO VCO frequency (45.150 to 74.995 MHz), controlled
by a digital PLL. The 2nd LO at 44.545MHz +/-2.5khz is
a varactor tuned VCO.

To generate products every 5kHz, surely we need a 5kHz
signal or two very strong signals that are 5kHz apart
going into the mixer or some non-linear stage after the
1st IF filter. Am I seeing something unusual or is this
what I should see? If it's an anomaly, can anyone
suggest what the cause might be? I'm thinking this is a
fault or a design weakness that with more complex
signals gives rise to a lot of interference to good
reception. Could it be possible that this is the result
of the method of observation and is not really
occurring in normal practise?

73 Tom