[Premium-Rx] TenTec Rx340 vs HF1000A

[Michael O'Beirne]

Morning gents,

It's a gorgeous day here in London.

The Ten-Tec 340 was the subject of an extensive technical review in the 
RSGB's journal, RadCom, in March 2002 by Peter Hart G3SJX, who I rate as one 
of the fairest and most accurate reviewers around.

It gets excellent marks for AM quality (synchro demodulator) (the best he 
had tested up till then).  The filters were very good.  But there were some 
down points:-

1.    The bandwidth prior to the DSP is wide - about 16 kHz - and strong 
signals close to the wanted signal cause overload and AGC distortion.  It 
was not possible to measure reciprocal mixing at b/ws below 10 kHz.

2.    Below about 2 MHz the dynamic range drops drastically.  For example 
with the RF amp out and at 50 kHz tone spacing, the IP3 at 2 MHz is +35dBm 
and the DR is 104dB.  At 1.5 MHz these drop to +12dBm and 89dB.

Others contributors recently to PR have commented on the significant drop on 
the MW and LW.

I think the set has above average looks and a decent uncluttered front 
panel. With the limited experience of trying out one at an indoor rally some 
years ago I was impressed, but the tuning feel is miles below what I 
experience with my Racal RA1772 and 3701.  The problem is that the shaft 
encoder is tiny.  It's more the size you see on instrumentation eg to set 
the RBW on a spectrum analyser, not a big beefy device suitable for constant 
tuning.  There are only 42 steps per revolution, so tuning is painfully 
slow, really more appropriate for fine tuning.  In contrast many ham 
transceivers give 1,000 steps per revolution.  The tuning feel on the old 
Trio 930 transceiver was miles better.

My in-depth experience of DSP-based receivers is limited to the STC STR8212 
from the early 1990s.  It has superb DSP filtering and demodulation (though 
having only 13 bandwidths) and delivers stunning audio and I have 
experienced absolutely no signs of overload with my Wellbrook loop.  There 
is no residual audio hiss whatever when the AFgain is at zero and very 
little audio distortion.

The DSP is early clunky stuff using a Tektronics ADC (designed originally 
for their scopes).  This uses a pulse density modulator for digitisation. 
According the manual the single bit stream from the PDM is then applied to a 
simple SINC filter which averages the high number of input samples.  This 
increases the precision of the samples at the expense of decreasing the 
sample rate.  After this first stage of digital filtering the signal is 
subjected to a digital mix which converts the 50 kHz IF to 0 Hz IF with I 
and Q outputs.  These channels are further filtered by a 120 tap FIR filter. 
There is then further processing.

The filtering uses a FIR linear phase design to give no group delay.  Shape 
factors of better than 1.66 to 1 (1dB to 100dB) are achieved with bandwidths 
greater than 2.4 kHz.

I measured the IP3 at maximum sensitivity with a signal separation of 20 kHz 
at about +24dBm, which is pretty respectable.

The Ist IF image rejection measured was around 100dB, and it was very 
difficult to locate the 2nd iF image in the noise - well over 110 dB 
rejection.  This remarkable rejection (dropping from 70.05 MHz Ist IF to the 
2nd IF and the DSP at 50 kHz in one jump) is achieved via a complex image 
rejection mixer using MCL hybrid splitters and Si8901Y ic mixers, and 
followed by a  complex polyphase network.

This one hell of a complicated radio.

The general construction is excellent (but see later).  Top quality 
professional RF and power connectors.  Even the top lid has strips of 
gold-plated finger stock to ensure good screening contact.

There is no pass band tuning, notch filtering or noise reduction, which is a 
pity.

Unfortunately it has a similar poxy shaft encoder to the 340, and the tuning 
is not so good.  The one I have borrowed has a slightly wobbly tuning feel. 
You notice such things after years of tuning Eddystones and Racals - perhaps 
just getting fussy in old age!

There is also evidently an audio HPF somewhere inside restricting the bass 
response.  This makes little difference on CW and SSB but is noticeable on 
AM.  In contrast, the audio on my RA3701 extends to at least an octave or 
more below, and for me is preferable for AM except where the conditions 
demand the superior DSP IF filtering.

The frequency tuning spec is down to 150 kHz but it will tune down at lesser 
sensitivity and more noise down to zilch.  However it is less good below 50 
kHz.  For lower I use an ancient dedicated valved VLF upconverter, the Racal 
RA137, outputting on 2 - 3 MHz, which is fed to the RA1792 or 3701.

However, the two major drawbacks with the 8212 are the two very noisy fans 
and reliability - probably dry joints.  Every now and again the set fails 
its BITE test and stops playing.  Life is eventually restored after wiggling 
every PCB.  Only about 150 were made before STC were bought by Nortel, who 
killed off the production to concentrate on making telephone gear.  These 
issues might have been resolved with quantity production.

There is an extra empty card slot in the DSP stages for further processing, 
but this appear never to have been developed prior to the kill off, which is 
a pity because this receiver had good potential.

As for spares - forget it.  You are on your own!  The manual is massive and 
detailed but you will need specialist extender boards to gain access to the 
coax break out points since the PCB card sockets incorporate several coax 
connectors - possibly Cannon jobs as I have seen with Norlin and WJ gear. 
The circuits are computer drawn in the cramped style of the long dead 
American "Ham Radio" (I think), where wiggly resistors look just like 
inductors and transistors have no ring round them, and the numbers need a 
magnifying glass to read.  Why can't we still have decently drawn circuits 
as with the old Collins and Racal manuals?  Larger paper is not that 
expensive.

The prime buyers of the 8212 appear to have been the military listeners, 
particularly as the top frequency is 40 MHz.  Such guys would have installed 
the rigs in a rack in one room and remotely controlled them from PCs 
elsewhere, so that the fan noise irritation did not arise.

Hope this helps.

Michael
G8MOB