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Author Topic: unwanted opposite sideband suppression  (Read 5007 times)
KB2SIN
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Posts: 26




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« on: June 29, 2005, 01:39:38 AM »

can anyone  tell me how to figure my unwanted opposite sideband suppression,in db,  using 14.200 mhz as my frequency and an output of 4 watts? The formula to figure this out is what i need.
Thanks and 73's
de kb2sin
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HA5RXZ
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Posts: 380




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« Reply #1 on: June 29, 2005, 02:24:19 AM »

Unwanted sideband supression is something which you will have to measure, not work out. The easiest way to do this (but the most expensive) is to use a spectrum analyzer with a bandwidth narrow enough to see both sidebands and your supressed carrier.

This task can be done with a seperate receiver and a calibrated S meter but it is not easy. Transmit a steady tone on 14.2 MHz USB at low power and then tune the receiver until you can hear your transmission. You then have to use a set of attenuators to reduce the input signal to a more manageable level, say S9 +20dB.

If you then switch to LSB on your receiver and tune around you may be able to hear your unwanted sideband. If you measure a signal of S7 then your unwanted sideband is 32dB down (20dB plus 2 x 6dB per S point).

Do not of course plug your receiver directly into your transmitter or there will be lots of smoke.

HA5RXZ
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W9PMZ
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« Reply #2 on: June 29, 2005, 02:29:49 AM »

You need a spectrum analyzer to make this measurement and an attenuator (since most analyzers don't want to see more than 30dBm or 1 watt).

So, you setup the analyzer for, say 14.2MHz, USB, and feed the transmitter with a 1KHz tone.

There will be 3 signals of interest, the main tone, at 14.2MHz + 1KHz, the suppressed carrier at 14.2MHz and the suppressed opposite sideband at 14.2MHz - 1KHz.

Find the reading for the signal at 14.2MHz + 1KHz, say 4 watts (but most RF instrumentation will be in dBm or decibels referenced to 1 milliwatt) which is 36dBm and then measure the suppressed carrierr at 14.2MHz and the suppressed sideband at 14.2MHz - 1KHz.

In most instances the suppressed carrier will be further down than the suppressed sideband.

So the suppressed carrier reads -10dBm and the suppressed sideband reads 0dBm.

Therefore the suppressed sideband suppression is 36dBm - (0dBm) = 36dB (or, 36dBc below the carrier).

Continuing, the suppressed carrier is 36dBm - (-10dBm) = 46dB (or, 46dB below the carrier).

Conversely,

If your transmitter is rated at 40dB for carrier suppression; then the power of the suppressed carrier is: 36dBm (4 watts) - 40dB = -4dBm.  To convert this back to millwatts, 10 exp (-4 / 10) = 0.4mW or 0.0004W.

73,

Carl - W9PMZ
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KA5N
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Posts: 4380




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« Reply #3 on: June 29, 2005, 03:35:45 AM »

The question is what are you going to do with this information?  The amount of unwanted sideband suppression depends on the placement of the carrier on the slope of the filter response, the shape of  the filter response and the amount of attenuation of the filter.  With a 4 watt signal the unwanted sideband is not going to be very strong even if the placement and adjustment of the signal is poorly adjusted.  
If you suspect that there is a problem with sideband suppression, tune an AM station and with its carrier zero beat, alternately switch sidebands.  If the signal is equally strong in either sideband, then the carrier placement is probably ok.  Some filters are not symetrical and one sideband will always have more supression than the other.
Allen
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W8JI
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« Reply #4 on: June 29, 2005, 03:53:36 AM »

Myth alert !!!

S meters are not 6 dB per S Unit, except in very few receivers. In most receivers I've checked S units are as little as 1 dB or as much as 6 dB depending on the receiver and what part of the scale you are on.

Don't depend on S meters.

The proper way to measure signal levls with a receiver is to set the desired strong signal to a reference point like S5, and then remove attenuation until the weaker signal is S5. The difference in attenuation is the signal level difference.

Measuring a filter or phasing system with a spot frequency is a problem. Filters and phasing systems have ripple. The normal test is actually pretty sophisticated, and you should use a noise or voice test signal.

Why are you trying to measure sideband suppression?

73 Tom

 
 
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K0IZ
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Posts: 737




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« Reply #5 on: June 29, 2005, 06:12:41 AM »

If you don't have any test equipment (which is, of course, the best way to get your answer), try this:

Assuming you have a transceiver, in the receive mode, switch back and forth between LSB and USB while listening to a "dead" band (ie, just background noise).  If the pitch of the noise seems about the same, then your bfo freq is about right (ie in the center) for both sidebands vs filter passband.

Next, again on receive, tune a fairly strong carrier (calibrator signal would be perfect), maybe S9 to 20 over.  Tune to the "wrong" sideband and listen to the signal.  It will probably be much less in volume (and S meter) compared with the "right" sideband.  Assuming so, you can plot, using the S meter, the strength of the carrier in the wrong sideband, starting at very low pitch and going higher.  

As pointed out above, most S meters are not very accurate.  S units can vary from 2 or 3 db per S unit to maybe 10db, and will likely vary at different points on the meter scale.  But doing the plot will give you a fairly good sense as to how the "wrong" sideband compares with the "right" sideband.

Having done all of this, you can likely assume that your transmitted signal on the "wrong" sideband is similarly reduced.

Even only 30db sideband suppression, for your 4 watts, would represent only 1/1000 of 4 watts.

John
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HA5RXZ
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Posts: 380




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« Reply #6 on: June 29, 2005, 12:42:51 PM »

No panic here, I did say a CALIBRATED S meter.

HA5RXZ
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N9AVY
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Posts: 66




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« Reply #7 on: November 23, 2009, 06:53:00 PM »

Looks like someone got an OO Notice....
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KA9DFI
Member

Posts: 3




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« Reply #8 on: November 23, 2009, 06:53:32 PM »

The reason he wants this information is answer the questions on the ARRL Official Observer (OO) test.
I think this might be a 'no no.'
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G3RZP
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Posts: 4487




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« Reply #9 on: November 23, 2009, 11:30:25 PM »

In most cases, the 'S' in 'S meter' stands for 'suspicious'.
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