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

Fri Feb 11 19:17:19 EST 2005

I appreciate the general worry about the DDS spurs.

What I wonder is whether the general worry really describes our case. For 
example: we have a limited frequency range for the proposed MW-160m 
receiver. The frequency of the spurs can be determined once you know the 
reference clock, phase truncation error, DAC quantization error, and angle 
to amplitude mapping error. That means many of the spurs can be removed by 
fixed filtering. How much the remainder of the spurs are a problem is 
difficult to say without talking about the implementation.

In any case: given a fast, 12 bit DAC a la the AD9854, I think it is quite 
possible we can create an LO with excellent phase noise and spurs near 100 
dB down and hopefully better. That's a guess on my part without running any 
analysis. Anyone else with an idea?

Chuck

>From: "Jerry" <gh1lockett at bak.rr.com>
>To: <premium-rx at ml.skirrow.org>
>Subject: Re: [Premium-Rx] Receivers for MW and 160 meters.
>Date: Fri, 11 Feb 2005 12:12:07 -0800
>
>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
>
>
>
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