[Premium-Rx] Receivers for MW and 160 meters.

[Jerry]

I forwarded this thread of notes on to a friend of mine, Bill 
Carver-w7aaz, who has built more than his share of high performance 
receivers/transmitters over the years and whose basic IF strip is built 
into the CDG2000 high end homebrew transceiver project that has/is being 
built by a lot of folks..  He had this to say:


I looked at that "PTS" synthesizer. It looks fine for phase noise,
obviously a DDS-based design. But, like every other DDS, they have
inband spurs. Those are specified as -75 dBC.

So your receiver, in addition to being tuned to the desired frequency,
is also tuned to a myriad of other places and down 75 dB. Would you be
impressed by an xtal filter with skirts down 75 dB? It would work, it
would sound good, but the first time someone 10 KHz away moved your
S-meter you'd be pissed. That sounds like my 75A4, cica late 50's, with
mechanical filter blowby.

According to data sheets a well-filtered pair of AD9951s in quadrature,
at HF, should produce an output that's comparable to the PTS. Either is
fine for proof of concept and casual, but it is  NOT  providing
performance of a premium receiver. We need a synthesizer design that
makes an LO with spurious down at least 100 dB to be acceptable, 120 dB
to be considered "premium" receiver performance.

Karl-Arne, SM0AOM, proposes a receiver that should have reasonable
performance. It is totally dependent upon (1) synthesizer cleanliness
and (2) the 24 bit ADC. All the emphasis is on digital today, the 24 bit
audio ADC will get nothing but better. The only undefined block is the
synthesizer:  what does Karl-Arne propose for that synthesizer? I am
ready to build it.

Pass that back. I don't need a detailed schematic, just a block diagram
with some performance numbers scribbled in.

Bill



Sent: Friday, February 11, 2005 7:09 AM
Subject: Fw: [Premium-Rx] Receivers for MW and 160 meters.


: Interesting comments..
:
: Jer
: ----- Original Message ----- 
: From: "Karl-Arne Markström" <sm0aom at telia.com>
: To: <premium-rx at ml.skirrow.org>
: Sent: Thursday, February 10, 2005 10:54 PM
: 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