Single or multiple conversion..............

[RolandSWL]

Are multiple conversion super-hets inherently superior to single conversion?

Can a single conversion radio be as good as a double or triple if it's properly designed?

Pros/Cons?

Thanks, Roland.............

[WQ9E]

There are a lot of "it depends".

A single conversion receiver with a lower frequency IF is going to suffer from image problems at higher frequencies unless the RF stages are unusually selective.  At 2 Mhz. using a single conversion 455 Khz. IF the image frequency (2.910 Mhz)  is nearly 50% removed from the desired frequency and even a simple front end will greatly attenuate the undesired image but at the other end of the range, 30 Mhz.,  the image (30.910 Mhz) is only about 3% removed from the desired frequency and the attenuation even with a two stage RF amplifier image rejection is going to be minimal.   Using an IF of 4 Mhz. would greatly reduce image problems (image frequency would now be around 30% removed from desired at 30 Mhz.) but you would need a crystal filter for decent selectivity.

Wide range receivers in recent years use up conversion to a low VHF first IF and then mix down to a lower IF where the bandwidth filters will be found.  

Classic higher performance receivers of the 50s, 60s, and 70s generally used at least two conversions with the first higher frequency IF providing good image rejection and the lower IF provided good selectivity.  A good example of this approach is the triple conversion Hammarlund HQ-170/180 which used a final 60 Khz. IF for sharp selectivity.   Even some receivers that provided most of the filtering at a higher IF provided a final low IF for additional gain and the addition of a sharp notch filter, the Drake R-4C, R-7, and Kenwood R-820 are examples of this approach.

Particularly for a limited range receiver a single IF at a fairly high frequency can provide very good performance and without all of the potential spurious responses that are generated through multiple frequency conversions.  The IF simply needs to be low enough that filters with good shape factors are practical.  

In any case having as much selectivity as possible before the high gain sections is very desirable and this was a major shortcoming of the multi-conversion designs like Hammarlund and Hallicrafters.  A lot of cross modulation and poor AGC characteristics came from this approach.

Of course now sampling systems are becoming very practical and that will be the next chapter in high performance design.

[ke7trp]

Thank you for posting that. I learned alot. 

One thing I was told, is that having an IF higher in frequency will be farther away from terrestrial noise.   The receiver would then be quieter.  My Yahoo runs 69.5mhz IF and has the filters running at that Frequency.  Its very quiet

The HRO50T runs two stages. BOTH at 455. This radio always confused me. They made it for a year or so.  Deaf on 10 meters.

C

[WQ9E]

Clark,

You are welcome.

I hadn't thought before about the higher IF frequency being quieter but unless the receiver is very well designed undesired signals can leak through directly into the IF as evidenced by the SX-101 problem I mentioned on the antenna trap thread.  Having a higher IF frequency with a sharp roofing filter that is not on/near commonly used frequencies does avoid this problem and there certainly isn't much atmospheric noise at 60 mhz.

The HRO-50T shouldn't be that bad on 10 meters; I have one but I don't think I have ever used it there.  For awhile it was the receiver I used with the Ranger/Desk KW but the selectivity wasn't good enough when I was operating as a net control on 75 meter AM and it got replaced by a SX-100 which was later replaced by a SX-88.  I bought a HRO-50-T1 with the additional IF amplifier stage and more selective IF transformers which provided better selectivity but shortly afterward the SX-88 came along.

[ke7trp]

Thanks for the reply. 

I just added a 2.4 network sciences 8pole Roof filter in place of the factory 3 on the yaesu. The factory had almost no improvment over the 6. The plot of this actual filter is included and shows 70+ db.  Not bad. But not cheap.   

I think my comments on the 50T are mainly because I compare the radio directly to my HRO60.  They are not even in the same ballpark on 15 meters.  20 to 30DB over signals vs the 50T being S3 to S4.  Sure, I can hear the "most" stations on the 50T but its nothing like the hotrod 60.  It has gain to spare. 

The 50T is good for Hifi listening.  It has a much wider bandpass of almost 20KC. Where the 60 is 5kC at its widest setting. 

Thanks again.

C

[KM1H]

One thing I was told, is that having an IF higher in frequency will be farther away from terrestrial noise.   The receiver would then be quieter.  My Yahoo runs 69.5mhz IF and has the filters running at that Frequency.  Its very quiet

Huh?

There is no external noise at the high IF if the front end has properly estabished the noise figure to well below the terrestial. The only reason for VHF IF's is to accomodate an almost DC to HF or 6M synthesized general coverage radio.
Otherwise a single conversion at say 9MHz is all that is needed BUT getting good selectivity there for CW and even steep SSB skirts wasnt possible at an affordable price. The Kenwoods, etc, solved that problem to a high degree but still need filtering at 455 to almost brick wall the skirts especially for CW bandwidths.
Companies like INRAD take it a step further by contracting with Japanese filter manufacturers to tighten up the tolerances from what is mass produced for their customers.  My 400 Hz INRADS are better than 250 Kenwoods for DX pileups and noise digging.

Carl

[ke7trp]

I am going to have to disagree here.. I am in the shop today and I have a military sig gen that goes way up to 400mhz.  I tuned around and various receivers can hear the IF Frequency very strong.. I think there is a real advantage to having an IF freq in an area that there is less man made noise. 

C

[Steve - K4HX]

What was the output level of the signal generator? And what antenna was it connected to?

Think about this. If external noise can get into the IF, why wouldn't TV stations get into the IF. Many up converting transceivers have a 60-70 MHz first IF. The TV stations are  thousands of times stronger than the noise.

[KM1H]

I tuned around and various receivers can hear the IF Frequency very strong.. I think there is a real advantage to having an IF freq in an area that there is less man made noise.

I dont understand what your trying to say. The IF is just an amplifier stage and doesnt emit any signal as does an oscillator. Sig gens are notorious leakers as are cables. And with tube gear the lowest noise is at the lowest frequencies and not VHF.