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Improving 9k6 reception

Hello all,

to use the YAESU FT-736R on 9k6 packet like UO-22, KO-23, KO-25 and
others to come as well as on terrestrial packet radio networks it is
neccessary to modify the radio the one or other way. Several
informations have been published during the recent years. The main
info came from James Miller G3RUH.

Since recently the German manufacturer Symek, who is known by his
TNCs, is offering a small printboard with an IF FM demodulator to install into
the FT-736R (or also in other radios). This unit is available through
AMSAT-DL-Warenvertrieb which is operated by myself, and I will be
happy to tell you indiviudally how much (or little) it costs. 

If youlike to receive a quotation please send an eMail direct to me at
dj1km@amsat.org with the subject: Symek. I will send you price
and shipping info in return then.

What this unit can do read from the following report by James
Miller, G3RUH. I will not include in this mail the below mentioned
figure 1 which is a plot.gif of 6308 Bytes. If somebody wish I will
send this to him individually provided that he is able to receive
attached binary code.

Suffice to say that im using this module on my own FT-736 with great
success, having not noted the recent frequency shift of KO-23. I also
have tried it with a "normal" FM transceiver like YAESU FT-711RM, and
it works fine as well.

73 Reinhard from Hannover

                     Symek's IF FM Demodulator PCB
                      for 9600 baud Packet Radio

                        by James Miller G3RUH

9600 baud packet radio uses FM radios.  For reception, audio is taken direct
from the frequency discriminator and passed to the modem.  With most radios
this works very satisfactorily.  But what if you have a radio which is less
than good?  Here are some possible reasons:

  - IF bandwidth is too narrow
  - Insufficient AF bandwidth
  - Amplitude non-linearity
  - Excessive phase distortion
  - Little or no tolerance to mistuning
  - Any or all of the above

Or you might prefer the modification to your radio to be non-intrusive, non-
destructive, and demodulate independently from the existing IF.

A perfect solution to these issues is available from Symek GmbH, their product
called "ZFD-A".  It's a small PCB that features a no-frills 455 kHz FM
demodulator specially for use in 9600 baud packet radio systems.

You use it with your standard FM radio; pick up the 455 kHz signal after the
final mixer and before the radio's own IF filters.  The unit's output is AF
which has had none of the usual de-emphasis etc.

The demodulator is designed around the Motorola MC3371 integrated IF amplifier
and FM demodulator plus a muRata SFH455B ceramic filter.  There's about a
other discrete components.

The SFH series has an improved phase response compared with the more usual CFW
types, and are intended for digital communications and mobile telephones.  The
type "B" has a 6 db IF bandwidth of 30 kHz.

The frequency discriminator curve is shown in figure 1.  The working area is
very wide, some 25 kHz;  third harmonic distortion of a 6 kHz pk-pk deviation
sinewave is a negligible 0.16%.  The demodulator audio frequency response
extends to well beyond 10 kHz.

Sensitivity is 0.14 v/kHz, so a normal +/- 3 kHz deviation packet signal will
produce 800 mv pk-pk output.

The PCB is small enough (37x37mm) to be slipped inside the radio.  It requires
7v to 20v supply at 6 ma.

In addition to laboratory tests, I monitored the 9600 baud digital satellites,
and my local packet network using an FT736R with the PCB connected to diode
on the RX-unit.

The performance of this FM demodulator is outstanding.   Signals could be
mistuned by as much as 9 kHz and still decode clearly.  The frequency-voltage
characteristic is exceptionally linear, both static and dynamic, delivering a
perfect replica of what was originally transmitted.


The author is designer of a widely used 9600 baud packet radio modem.  The
technology is licensed to a number of manufacturers, including Symek

Many thanks to Reinhard Richter DJ1KM for calling my attention to, and the
of, this equipment.


Figure 1. Symek FM IF Demodulator Voltage-Frequency characteristic showing
range and excellent linearity at the operating point 455 kHz.
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