Ten Tec RX 340 Audio Response

[Steve - WB3HUZ]

There was a discussion about this receiver a while back on the Forum. I said then that I would make some frequency response measurements when I had a chance. I finally had a chance. As Larry/ER said, the audio response really does roll off quite fast. There no way you get the full utility of IF bandwidths wider than 6-8 kHz with the audio roll off. I also checked the IF output and the filter is pretty flat out to 16 kHz (8 kHz modulating frequency on the AM input signal). So the problem is definitely in the audio section. Worse yet, the low end rolls off too much for my liking on the headphone output. Errrr.

[wavebourn]

typical "Communications receiver".

[w3jn]

Either someone at TenTEc has been reading too many old ARRL Handbooks, or they're rolling off the highs to mask noise from the DSP and/or first audio stages.  

My Cubic R-3030 has a big problem with audio hiss in the AM mode.  According to Frank GFZ the problem is in the final IF stage, where there is a MC1350 amplifier running full bore gain.  Although IF noise most likely isn't the problem with this DSP rig, there might be artifacts and/or noise at the DSP output that they're trying to get rid of, because that drop off is AWFULLY steep.

Get in tha' wid dat scope, boy, and look at the DSP output or audio amp input and see what be happenin there.

73 John

[W8ER]

W3JN,

It was rumored to be aimed at reducing the hetrodynes from 5 khz channel spacing on HF shortwave AM broadcasts. That is speculation and rumor however. Who really knows why. The Drake R8B has an even steeper low pass filter for that very reason.

I don't think they are trying to mask anything. The DSP is clean as a whistle as proven when the Sherwood SE3 is added!

Steve,

If you get a chance, try the Sherwood SE3. It fixes the problem of the rolloff in the audio section and adds a great synchronous detector. The SE3 takes the 455 khz IF signal and gives you 5 watts of nice audio, suitable for driving a good speaker. Not that you should have to do something that drastic after buying a "lab" grade instrument.

The mod that I sent you corrects the rolloff but you are still stuck with the TenTec SAM mode and the rather poor audio section.

Good audio .. I liked the sound from the Racal RA-6790/GM but my HQ-145 still is the bang for the buck!  :cool:

-- Larry W8ER

[w3jn]

Larry - you need to try a SP-400!  MUCH better audio than my 6790/GM, HQ145, Cubic R-3030, SX-88, etc.  I haven't had the pleasure of using a Sherwood, but I'm in the parts gathering stage of building something similar using WA1SOV's all-pass filter designs.  Couple of NE602s, some Burrr-Brown precision opamps, a handful of .1% resistors, and I'm there!

The 5KHz heterodyne problem should be taken care of in the DSP, not the audio.    If the op is observing 5 KHz heterodynes, then (especially with a professional grade receiver) the op should know to narrow down the bandwidth.  To take the choice of hearing wider audio away from the op is an extremely poor engineering design choice, IMHO.

16 KHz DSP bandwidth should reproduce up to 8 KHz well, as steve noted looking at the IF output.  The Sherwood, however, if you have it connected to a 455 KHz output, is probably BEFORE the DSP.  Although I haven't looked at a skizmatic for this radio, DSPs need I/Q inputs and this is usually resolved by two product detectors driven by a LO at the last IF freq (one detecotr's LO input has been shifted 90%).  Some can do their processing at higher "IF" frequencies, but 455 KHz is getting up there towards the limit.

73 John

[W8ER]

W3JN said

The Sherwood, however, if you have it connected to a 455 KHz output, is probably BEFORE the DSP. Although I haven't looked at a skizmatic for this radio,

Actually John it is after the DSP. There are two 455 IF outputs on BNC's on the rear apron. One is before the DSP and one after. I am connected to the one .. after .. obviously because I want to use the DSP to control the bandwidth to tackle QRM etc.

I think like you that a 5 khz DSP notch should have been considered instead of the filter they designed in. In a receiver billed as a lab grade instrument, there should have been no corner unturned to be sure that it was done right!

What is sad is that they appear to be so busy with the Orion and maybe even the successor to the RX-340 that they did not address the problem when it was brought to their attention, other than to offer a mod to disable the filter. It's sort of, NRSC .. what's that?

I would really like to be founded well in American products instead of the Kenmores and such but TenTec got my attention with the Jupiter and the RX-340 and then I sit here looking at bugs and getting pissed because I paid their price!

I suggested one time that they open source the firmware and boy did that get shot down quickly! Can you imagine, some sharp firmware cookie grabbing the source and making those radios sing? I'd buy six of each just because but that's not about to happen.

--Larry

[Steve - WB3HUZ]

JN is right. If heterodynes are problem the IF filter should take care of it. That's the beauty (or could be) if the nearly continuosly variable IF BW, is that you can dial up  the BW for the given QRM. Larry is also right, if the notch was able to be engaged in the SAM mode and able to be tuned past 2 kHz, once again the hets would not be a problem.

I just can get the logic of having a RX that has an IF BW of 16 kHz and not an audio section that works to at least 8 kHz. Why not vary the AF BW comensurate with the IF BW.

Ah, well, one would expect better from a $4k RX. But I still don't see this thing as a pro receiver. It's way to cheap/inexpensive.

[w3jn]

How come you ain't watchin the "Big Game" (don't wanna get G in trouble using the trademarked actual name) there, Sizzlechest?

QTF with the 16 khz bandwidth indeed.

I will say it appears to be much more of a radio than the R-9000 but not quite in the class of the WJ 8711/HF-1000.

73 John