Receivers.

[Geoffrey Mackenzie-Kennedy-2]

Morning Bill,

It never ceases to amaze me how receivers (and antennas) attract so much
comment. Is it because the majority of us are using receivers designed for
the amateur market as benchmarks, or is it something that results from
habit? I suspect that the "Money Men" get involved closely in the design

My $0.02 worth for ham band only coverage:

1) Dual vs Single conversion. Both can be a disaster if the IFs are not
chosen properly  With the IFs chosen properly, and the receivers built
properly, up-conversion to the first IF with the LO on the high side, there
are far far fewer spurious responses to be found in a dual conversion than
found with a single conversion receiver, and "weaker".  The RF preselector
in a "Dual" has something more to add to attenuation. 'Built properly' is
very important.

Narrow first IF filters are essential in double conversion for "strength"
reasons, selectable by reed relays not diodes. Here I use VHF 12 poles, 6,3
and 1.5kHz.

2) Effect on Noise Floor.  Assuming the use of strong mixers (+50dbm) and
strong low noise figure IF(s), the difference can be zero.

3) LO purity. Both require a low phase noise LO(s), free of spurs. Until a
cheap low phase noise PLL appears, I'll stick with premix systems running at
VHF. Much more work, but worthwhile.

4) DSP. I fully agree. I see little point in having a ho-hum filter at the
front end of the IF, letting a cocktail of signals romp down the IF to be
dealt with by a DSP module. Great will be the day when a DSP unit running at
VHF, with a high IP3in etc, and low noise figure becomes practical.

73,
Geoff.     GM4ESD

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[N8LP]

I agree, and eventually RF/IF DSP will be commonplace... but you will
need a magnifying glass to see the chips, since all the good new stuff
is micro-miniature ;-)

Larry



Geoffrey Mackenzie-Kennedy wrote:

>Great will be the day when a DSP unit running at
>VHF, with a high IP3in etc, and low noise figure becomes practical.
>
>73,
>Geoff.     GM4ESD
>
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[Bill Coleman-2]

On Mar 7, 2005, at 8:51 AM, Geoffrey Mackenzie-Kennedy wrote:

> 1) Dual vs Single conversion. Both can be a disaster if the IFs are not
> chosen properly  With the IFs chosen properly, and the receivers built
> properly, up-conversion to the first IF with the LO on the high side,
> there
> are far far fewer spurious responses to be found in a dual conversion
> than
> found with a single conversion receiver, and "weaker".  The RF
> preselector
> in a "Dual" has something more to add to attenuation. 'Built properly'
> is
> very important.

There are a lot of up-conversion designs of this form. The biggest
problem with them is that the early stages are easily overloaded by
adjacent strong signals unless the first filter is very narrow. Narrow
filters at such higher frequencies are harder to build and more
expensive. Many design simply use a 15 kHz wide resonator.

> Narrow first IF filters are essential in double conversion for
> "strength"
> reasons, selectable by reed relays not diodes. Here I use VHF 12
> poles, 6,3
> and 1.5kHz.

The TenTec Orion and the IC-7800 use a similar strategy. However, they
use even narrower filters -- like 500 Hz for CW. This greatly improves
the adjacent signal rejection.

> 2) Effect on Noise Floor.  Assuming the use of strong mixers (+50dbm)
> and
> strong low noise figure IF(s), the difference can be zero.

Noise floor isn't a terribly big deal with an HF rig. It's not hard to
design a rig with a noise floor lower than atmospheric noise.

> 3) LO purity. Both require a low phase noise LO(s), free of spurs.
> Until a
> cheap low phase noise PLL appears, I'll stick with premix systems
> running at
> VHF. Much more work, but worthwhile.

Lots of work has been done in the last two decades to make low noise
PLLs, ever since the specter of phase noise raised its head with the
new general coverage transceivers.

> 4) DSP. I fully agree. I see little point in having a ho-hum filter at
> the
> front end of the IF, letting a cocktail of signals romp down the IF to
> be
> dealt with by a DSP module. Great will be the day when a DSP unit
> running at
> VHF, with a high IP3in etc, and low noise figure becomes practical.

All these things are theoretically easy -- just expensive with current
DSP hardware. So long as Moore's law holds, that should change in the
future.

Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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[KK7P]

>> ...Assuming the use of strong mixers (+50dbm)...

Hmmm.  +50 dBm = +20 dBW = 100 watts!  That's some mixer!!!

>> ...Great will be the day when a DSP unit running at VHF,
>> with a high IP3in etc, and low noise figure  becomes practical.

An interesting approach to this is the SDR-14 from rfspace.  It directly
  digitizes the entire HF spectrum -- and by using input bandpass
filters can digitize up through the 222 MHz band.  I have listened to 2
meter signals on mine.

The front end isn't as strong as a contesting station would need -- yet.

Meanwhile, for high-performance, a strong front-end followed by a
crystal filter suited to the mode and signal level, enhanced by a DSP
seems to be the best practical implementation.

Lyle KK7P

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[Geoffrey Mackenzie-Kennedy-2]

Morning Lyle,


Lyle Johnson KK7P wrote:


> >> ...Assuming the use of strong mixers (+50dbm)...
>
> Hmmm.  +50 dBm = +20 dBW = 100 watts!  That's some mixer!!!
>

          Could never get a job typing !  It should have read '( with an
IP3in of +50dbm). The mixer is the H Mode mixer developed by G3SBI some
years ago, and is a real performer.

> >> ...Great will be the day when a DSP unit running at VHF,
> >> with a high IP3in etc, and low noise figure  becomes practical.
>
> An interesting approach to this is the SDR-14 from rfspace.  It directly
>   digitizes the entire HF spectrum -- and by using input bandpass
> filters can digitize up through the 222 MHz band.  I have listened to 2
> meter signals on mine.
>
> The front end isn't as strong as a contesting station would need -- yet.
>

          That is interesting, and they will get there.


> Meanwhile, for high-performance, a strong front-end followed by a
> crystal filter suited to the mode and signal level, enhanced by a DSP
> seems to be the best practical implementation.

          Agree, provided the LO(s) very close in phase noise does not
compromise the filter, or at any further out offset the phase noise does not
compromise the dynamic range (PNGDR).

Tnx es 73,

Geoff.
GM4ESD

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[Geoffrey Mackenzie-Kennedy-2]

On March 08, 2005, Bill Coleman wrote:
>
> > 1) Dual vs Single conversion.

> There are a lot of up-conversion designs of this form. The biggest
> problem with them is that the early stages are easily overloaded by
> adjacent strong signals unless the first filter is very narrow. Narrow
> filters at such higher frequencies are harder to build and more
> expensive. Many design simply use a 15 kHz wide resonator.
>
          I would suggest that part of the problem is driven by cost, not
only by the production cost of a narrow bandwidth first IF filter but also
that of the "post filter" amplifier and 2nd mixer which should also be
'strong'. In my experience the wide bandwidth monolithic filters often found
at the head end also often yield poor IP3in performance, compromising to
some extent overall receiver performance.

> > Narrow first IF filters are essential in double conversion for
> > "strength"

>
> The TenTec Orion and the IC-7800 use a similar strategy. However, they
> use even narrower filters -- like 500 Hz for CW. This greatly improves
> the adjacent signal rejection.

          By using strong push-pull amplifiers and strong mixers right along
to the 2nd IF filter, in this particular receiver the 1.5kHz bandwidth tight
skirted 1st IF filter has not yet caused problems on CW. There is space
assigned for a narrower filter, and without question a narrower filter would
reduce the workload placed on the following stages. So far they have coped.

> > 2) Effect on Noise Floor.
>
> Noise floor isn't a terribly big deal with an HF rig. It's not hard to
> design a rig with a noise floor lower than atmospheric noise.

          Absolutely agree, provided the IP3in is maintained better than
that required.

> > 3) LO purity. Both require a low phase noise LO(s),

> Lots of work has been done in the last two decades to make low noise
> PLLs, ever since the specter of phase noise raised its head with the
> new general coverage transceivers.

          Some very recent work done in a country East of here shows
promise. Very difficult to get information.

> > 4) DSP. Great will be the day when a DSP unit  running at
> > VHF, with a high IP3in etc, and low noise figure becomes practical.

> All these things are theoretically easy -- just expensive with current
> DSP hardware. So long as Moore's law holds, that should change in the
> future.

          Indeed, I hope that they can get the noise figure, IP3in etc
right.

73,
Geoff.
GM4ESD

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