[aprssig] Re: Help setting up a new TM-D710 for APRS

Stephen H. Smith wa8lmf2 at aol.com
Wed Jan 2 13:16:46 EST 2008

Steve Noskowicz wrote:
>   Thanks, but would you answer my two specific  questions about the S-Meter ?  
>  I'm considering a hybrid and am interested in the types of noise.  
>   At these higher frequencies, it can also be harmonics of the switch-mode
> drive, which isn't necessarily stable enough to have audible birdies at the
> zillionth harmonic.
>     Which prompts me to ask if you know the switching freq these operate at?
>    Desense noise is indeed similar, but it can have an audible characteristic
> to it (that I can hear).  Receiver desense can come from the offending
> transmitter's sideband noise, or the receiver's own  Lo  sideband noise
> whichever is the worse (or their sum).

Again, the characteristic of all these noise sources is that the noise 
is completely broadband - no discrete frequencies.  The type of receiver 
desense I was referring to (regardless of whether it comes from TX 
sidebands or or a noisy RX LO) is the overlaying of "hiss" on a signal 
that would otherwise be fully quieted in the FM receiver --  the net 
effect is that the victim signal sounds as though it is reaching the 
receiver at .2-.3 uV instead of 5-10 uV.

I have listened to it with my Yaesu FT-100 "DC-to-light" transceiver in 
AM, FM and SSB modes. In all modes, you don't hear any discrete tweets, 
buzzes, squeals or grinding "hash" - just "hiss" at a slightly higher 
level than with the antenna input terminated in a dummy load.  In 
addition, you DO hear an additional pulsing "grinding" noise, on SSB or 
AM when the regenerative braking system is active, using the wheels to 
drive the electric motors as a generator.  This noise is barely 
noticeable on FM where it tends to quiet just like ignition spikes.

The conspicuous lack of discrete "lumps" of noise make me think it is 
NOT artifacts or harmonics of the switcher frequencies, but rather 
broadband noise generated by heavy DC current flows through 
semiconductor junctions.  I.e. the same effect as diode noise generators 
traditionally used to align VHF and UHF receiver front ends for optimum 
noise figure but on a much larger scale. 

On the HF bands, it masks receive with a constant S5-S7 noise on the 
FT-100's bar graph "S-meter".   On 2M, it would make the first segment 
of the Kenwood D700's bar graph S-meter flicker occasionally.   On UHF, 
there is no noticeable effect.  

The HF antenna (ATAS100) and VHF/UHF antenna (Comet 770 end-fed 
no-ground-plane-required 1/2-wave design) were mounted on the sides of 
the rear hatch opening. Details are (and the rest of the installation in 
my Prius) are shown here on my website:



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
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