Current bugbears

Jim Thompson jim at
Tue Sep 4 20:18:11 CEST 2007

On Sep 4, 2007, at 8:03 AM, Shawn Rutledge wrote:

> On 9/4/07, Richi Plana <myopenmoko at> wrote:
>> Fascinating. So "passive receivers" really aren't? Or are there  
>> classes
>> of receivers which are (no amplification or very sensitive pickups)?
> In theory nowadays DSPs are fast enough to process the lower bands of
> radio signals directly off-the-air with no IF, but I'm not sure if
> anybody's doing that yet.  Suspect the modern ham HF rigs might be
> doing that, but haven't researched it.  But a clock signal for any
> digital system might also cause radiated interference, right?  Radar
> and GPS and GSM are all too high in frequency to handle that way, so
> far.

John described heterodyne receivers, where the signal is first down- 
converted to intermediate frequency (IF) for efficient channel  
selection, and then filtered to remove interfering signals and  
amplified for another step of down-conversion to the baseband  
frequency. The heterodyne receiver provides good performance in terms  
of channel selectivity and sensitivity, but with the need for off- 
chip surface-acoustic wave (SAW) filters in combination with  
additional IF circuitry. Since SAW filters are fabricated using  
different material technology, they cannot be integrated with the IC.  
To replace expensive SAW filters in the heterodyne receiver, another  
approach is used- the low-IF architecture, where the RF signal is  
converted to a very low IF frequency, and then filtered using passive  
or active in-circuit filters. However, this architecture suffers from  
poor performance in terms of image rejection and channel selectivity.

The direct conversion (or homodyne) receiver eliminates the  
intermediate frequency stage, and directly down-converts the RF  
signal to a baseband signal. Without the IF stage, bandpass filters  
and other elements can be eliminated to reduce the bill of materials,  
and therefore cost.  Until quite recently, direct conversion issues  
have frustrated RF engineers with inherent problems such as self- 
mixing, 1/f noise and the resultant loss of sensitivity. Self-mixing  
comes from the LO signal making its way to the input of the mixer,  
which generates DC as a mixing product, possibly saturating the  
following filters and gain amplifiers.  The sensitivity problem and 1/ 
f noise have been solved by optimally controlling (normally via DSP  
or a small CPU), a number of amplify-and-filter stages in the  
receiver chain.

You likely have a direct-conversion receiver nearby right now.  Its  
your Wi-Fi card, unless you're still using 802.11b, or you've got an  
Atheros card (but I don't want to go into that too deeply.) It was  
explicitly the cost-reduction driver for BOM reduction which forced  
the WiFi market to direct-conversion receivers.

>> Prolly off-topic, but I sure am curious. Are there no radar detectors
>> which don't give off their presence?
> says that first radar
> detector detector detectors were developed so the radar detector can
> shut down when it detects that it's being detected; then they just
> started using a lot of different IF's so the police can't monitor them
> all.  So modern radar detectors are mostly immune to detection.
> Yes it's off-topic... will shut up now

Valentine One, there is no substitute.


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