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Introduction

I've discussed how to make intermodulation measurements using the Z90, a high power attenuator and a two-tone generator at the parent page. High power attenuators are not commonly found in the average ham's shack, so I've added this page to show how Telepostinc's LP100 wattmeter coupler may be used to provide the signal sample.

Although it is most useful when used with an auxiliary low-power attenuator, it's possible to measure a K2's intermodulation performance and carrier suppression up to about 2 watts output without an auxiliary attenuator.

 
The Current Sample output port from the LP100 wattmeter coupler yields a signal sample 34 dB below the input power.

Use the Current Sample output for the signal sample. You may either leave the Voltage Sample port open or terminate it with a 50 ohm load. If you terminate it with a 50 ohm load, make sure it has adequate power rating based on the test transmitter power. The Voltage Sample port is also 34 dB below the input level.

 
Discussion

The diagram below shows how to connect the Z90 to the LP100 coupler to perform IMD and carrier suppression measurements.

Equipment Required:

  1. K2 or other transceiver
  2. LP100 Wattmeter
  3. Two-tone generator, capable of operating in both simultaneous two tone mode or single tone mode. If you don't have a hardware two-tone generator, you might try the software version for Windows "dgen" at http://www.dxzone.com/
  4. Strongly recommended -- auxiliary 20 dB attenuator. If you intend to operate at 100 watts or less transmitter output power, the attenuator should be rated at 100 mw or greater power.
  5. Dummy load
  6. Z90 or Z91, and recommended Z90-Control software for screen captures

Please pay close attention to computing the maximum safe transmitter power. It is possible for you to damage the Z90's input stages if excessive power is applied.

 
   
Let's work through the maximum safe power computation.

I'll assume you are familiar with expressing power in dBm, i.e., dB above (positive dBm) or below (negative dBm) 1 milliwatt. 1 watt is thus +30 dBm; 100 watts is +50 dBm and 1 microwatt is -30 dBm.

The Z90's maximum signal level (set by linearity considerations) is -30 dBm. (For better performance, keep the peak signal level to -40 dBm.)

If the Z90's internal attenuator switches are set to maximum value (30 dB), the recommended power level into the Z90 is 0 dBm.

The LP100 coupler has a 34 dB coupling coefficient. The K2's maximum output power is thus +34 dBm unless an auxiliary attenuator is used between the LP100 coupler and the Z90. +34 dBm is 2.5 watts.

Inserting an auxiliary attenuator between the LP100's Current Sample output and the Z90's input increases the maximum possible test transmit power dB for dB. A 20 dB auxiliary attenuator therefore increases the safe transmitter power to 250 watts (+54 dBm).

Test procedure:

  1. Compute the maximum permitted output power. Check your calculations. If you do not have an auxiliary attenuator, the maximum output power you will be able to use for the test is 2.5 watts.
  2. Connect the LP100 wattmeter to the dummy load and the K2 for normal use. Set the K2 levels so that you will not exceed 2.5 watts or whatever power you compute in Step 1. When setting the power level, use the  two-tone generator set for single tone mode as the modulation source.
  3. Set the Z90's attenuator switches to 30 dB (both switches pushed IN).
  4. Connect the Z90 and two tone generators as shown in the above diagram. I'll use illustrations for LSB, but you can make similar tests in USB.
  5. Set the K2 to 14.300 MHz, or whatever other test frequency you wish to use.
  6. Program the Z90 with a "custom IF" corresponding to the test frequency.
  7. Set the Z90 to either 5 KHz or 10 KHz span, 200 Hz RBW, averaging = 1, skip = none, scan speed = auto.
Test A--Carrier suppression and unwanted sideband suppression

Set the two-tone source to single tone (1 KHz usually) and key the transmitter with the transmitter output set to the maximum power level computed in Step 1 above. You should see something similar to the image at the right. The carrier may be merged into the wanted sideband in your  transmitter.
   

 

Note the amplitude of the desired sideband signal. In the example, it is +5.8 graticule units, or 58 dB above the bottom of the display box. Note the unwanted sideband level. It is +2.3 graticule units, or 23 dB above the bottom.

The unwanted sideband rejection is thus: 58 - 23 = 35 dB.

 
If you see many signals, such as in the image to the right, you are over-driving the transmitter. You should reduce the audio level and or other level adjustments until you see a trace more closely resembling the upper image.
   
To measure the carrier suppression, remove the two-tone generator from the microphone input and key the transmitter. You should see the carrier only, as seen to the right,.
 

The carrier is seen at +1.3 divisions above the bottom, or 13 dB. Thus the carrier suppression is 58 dB - 13 dB = 45 dB.

 
Test B. Measuring Intermodulation Distortion

To measure intermodulation distortion:

  1. Reconnect the LP100 as a wattmeter.
  2. Switch the tone tone generator to two tones. Set the tones to 700 and 1900 Hz, if the frequencies are adjustable.
  3. Connect the two-tone generator to the K2's microphone input and set the transmitter power (PEP) to the value calculated in the earlier steps.
  4. Turn the two-tone generator off and reconfigure the test setup as for Test A.
  5. Set the Z90 to 20 KHz span, 200 Hz RBW, Skip = None, Avg = 1, scan speed = auto
  6. Turn the two-tone generator on and observe the output on the Z90.

 

   
This is a typical output. Note that the two main test tones are equal amplitude. If your test tones are not equal amplitude, adjust the balance or levels on the two-tone generator until the two are essentially identical.
   
In the example shown above, the 3rd order intermodulaton product is approximately -35 dB with respect to the single tones, and the 5th order IMD product is also about -35 dB with respect to a single tone.

Since the PEP is 6 dB above the single tone value, the IMD referenced to PEP is -41 dB for both 3rd and 5th order products.

Some manufacturers specify the IMD as X dB below PEP. This inflates the IMD suppression by 6 dB.

The data presented on this page was generated with a Telulex SG-100 digital function generator in LSB mode, and the two test tones were generated with dgen software.

If you over-drive the transmitter, the IMD products will rapidly increase. Hence you may wish to experiment with different audio and drive levels to see how IMD varies with drive.

One additional caution is that some sound cards and computers have a lot of low frequency noise, 60 and 120 Hz audio and this may get into the audio if you use dgen or similar software. Accordingly, I strongly recommend a good hardware-based two-tone generator for serious IMD measurements. Elecraft's 2T-gen seems to be a suitable two-tone generator.