[Premium-Rx] Receivers for MW and 160 meters.
Chuck Hutton
charlesh3 at msn.com
Fri Feb 11 02:33:27 EST 2005
That's my dream receiver, and it is do-able. What's your LO design: a DDS,
or a DDS followed by a PLL to clean up the DDS spurs?
When do we start working on it?
Chuck
-----Original Message-----
From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org] On Behalf Of Karl-Arne Markström
Sent: Thursday, February 10, 2005 10:55 PM
To: premium-rx at ml.skirrow.org
Subject: Re: [Premium-Rx] Receivers for MW and 160 meters.
I certainly agree with prevoius posters that nothing can be substituted for
front-end selectivity.
Off-channel signals that have been eliminated through preselection simply
cannot cause problems downstream.
Proper choice of mixers (the SS-1R has been mentioned) helps a lot, and if
paired with preselection
and a low-noise oscillator as in the G3PDM receiver design the results can
be spectacular.
To take up previous discussions about diplexers, it is my firm belief that
lumped-constant
diplexers intended to smooth out impedance variations near the passband from
crystal or mechanical
filters would be impractical.
The exception may be the route taken in the E1700/E1800 where a 90 degree
hybrid absorbs the
impedance mismatch from crystal filtering behind the mixer.
A proposed design for a very high-performance MW and 160 m receiver would in
my opinion look
like this:
4 dB Cohn-filter preselector with a - 3dB passband of 0.5 to 1 % of the
centre frequency;
Push-pull MOSFET feedback low-noise amplifier, Gain 10 - 12 dB, NF < 2 dB
and IP3 > 50 dBm;
A passive power divider to the I and Q signal paths, each with a high-level
"Tayloe-mixer" driven from a
low-noise frequency synthesizer via high-speed logic I and Q LO drivers;
Passive low-pass filtering in each signal path in the mixer;
Low-noise I and Q baseband preamplification before 24-bit A/D conversion;
"Software Defined Radio" signal processing a'la the SDR-1000 software
downstream
Such a receiver, as modeled in HP/Agilent "AppCad", shows an NF of 6 - 8
dB, an IP3 with 10 kHz spacing of around
40- 45 dBm (30 kHz spacing IP would probably be impractically high to
measure with "normal" lab gear).
Should the ultimate in sensitivity be needed (for example when using
Beverage antennas in extremely quiet locations), the preselector and the
low-noise amplifier (preferably preceded with some filtering) could trade
places.
This would yield a receiver with a noise figure of around 4 dB and a dynamic
range of >110 dB in SSB bandwidths.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "w3jn" <w3jn at direcway.com>
To: <premium-rx at ml.skirrow.org>
Sent: Friday, February 11, 2005 1:22 AM
Subject: Re: [Premium-Rx] Recievers for MW and 160 meters.
> Brian, good points. Tight RF selectivity is ALWAYS good. The reason the
NF and dynamic range isn't that great on the HRO is due to its prehistoric
6K7 RF tubes, and the abysmal 6A8 (6B8?) mixer. A popular modification of
the day was to replace the 6K7 or 6K7 with a 6AK5, thereby improving the
noise figure by some 15-20 dB.
>
> One of the best mixers ever is the 7360 dual sheet-beam tube. The
Squires-Sanders SSR-1 used this tube as a mixer - and it didn't even need a
RF tube to achieve relatively good .2uV for 10 db S+n/n ratio, but you could
blast a couple volt signal 10 kC from a 10 uV signal and the weaker sig
wasn't even affected. I built a homebrew RX using this tube as a first
mixer, directly driving 1.4 MC xtal filters pirated out of a Racal 6217, and
its performance really isn't matched by anything else I've used. Any of the
DSP receivers I've used really don't hold a candle to some of the late tube
designs.
>
> 73 John
> ----- Original Message -----
> From: Brian D. Comer
> To: premium-rx at ml.skirrow.org
> Sent: Thursday, February 10, 2005 11:12 AM
> Subject: [Premium-Rx] Recievers for MW and 160 meters.
>
>
> I realize that most of us always want to have a receiver that does
everything but it seams to me that if you want the best performance at these
frequencies the high IF HF receivers are far from optimum.
>
>
>
> The problems in building narrow band filters at frequencies greater than
30 MHz are much greater than at lower frequencies. Phase noise increases by
the square of the frequency so using an LO that is about 40 times what is
needed makes no sense. The diplexers that have had all the discussion are
not realizable at 40 MHz with narrow band filters if one looks at close in
stuff. The only passive ways around this that I know of are complex and
expensive. Following the mixer with a broad band amplifier is the most
common solution to this problem.
>
>
>
> The point made by Micheal G8MOB that some of the older receivers using a
similar architecture to the HRO AR88 etc. may be a better choice is a good
one. However the HRO that I have has an IIP3 of 9 dBm and a noise figure 15
dB worse than that of my Orion with the RF amplifier turn off. This results
in about a 30db lower dynamic range than that of the Orion. I am certain
that a receiver made with this architecture using today's components would
make the best High IF HF receiver look very bad.
>
>
>
> Allowing for the fact that a lot of the noise at these frequencies is
aggravated by highly selective filters I believe that the best approach
today for these bands would be the use of constant delay filters for some
pre-selection and direct DSP.
>
>
>
> I think we have a bit of a dilemma that our interest in radios is
somewhat based doing things the hard way. We like to use these bands more
for their challenges than their advantages. We find it hard to part with
the feeling of the tuning knob that had to turn a four gang tuning capacitor
and hence needed some very nice anti backlash gears and a flywheel. Now we
have to put up with a shaft encoder that runs so free that a friction device
has to be added to make it useable. In the case of the Orion this has been
added to the knob and is probably partially responsible for the wobble.
Gone is the analog feel as we now have steps to deal with as a result of
poor shaft encoder resolution.
>
>
>
> At this time I have an eddystone EA12 and an Orion on my desk. Most of
my listing is on 160, still trying to learn the code after 40 years! If I
want to just play listening the EA12 feels nice to tune and is great fun.
When it comes to being serious the Orion stays on frequency, is easier to
tune, has better adjustable selectivity, is orders of magnitude better in
handling QRM and is about 50% computer.
>
>
>
> The Orion's main receiver has no roofing filters in the normal sense,
it simply has crystal filter IF selectivity at 9 MHz and DSP
>
>
>
> 73 Brian KF6C G3ZVC
>
>
>
>
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