I THINK I'M ON FREQUENCY, I THINK I'M ON FREQUENCY (wouldn't stay on frequency, that is!)

In mid-September of 2003, I bought a Kenwood TH-F6 handheld from HRO in Salem, NH. I was so impressed with its performance and ease of operation that I decided to buy a Kenwood TM-D700A mobile rig for my vehicle. I bought the radio (S/N 505000XX) in late September from HRO in Salem, NH. While familarizing myself with the radio on the test bench and checking out all the cool APRS/packet features, I happened to notice the radio was transmitting off frequency according to my service monitor. More testing over the next few days revealed it had a terrible drift problem on UHF! I drove back to HRO in early October, and they happily exchanged it for another one. The second TMD-700 had a serial number 44 units below the original one. I had hoped the second radio would solve the drift problem. Boy was I wrong! The second radio was no better than the first one! Over the next two weeks, I ran a battery of tests on the second radio to see just how bad it really was. The results of these tests are documented below:

The graph below shows the frequency stability of my new Kenwood TH-F6 hand-held. The test was conducted at room temperature (72-74 degrees F) with the HT transmitting on 446.000 MHz at the high power (5W) setting. The HT was keyed for 1 minute, then unkeyed for 2 minutes, resulting in a 33% duty cycle. Frequency readings were taken at the beginning and end of each transmit period. This was repeated for 60 minutes. The graph shows frequency error (in Khz) versus time (in minutes). The TH-F6 starts out 150 Hz low in frequency, and ends up 370 Hz low at the end of the test. This calculates to a frequency stability of .83 ppm, far better than the Kenwood 8 ppm spec! This is the type of performance one would expect of a modern, synthesized PLL based radio.

Kenwood TH-F6 HT drift at room temperature

Let's see what happens when the TM-D700A is subjected to the same test under the same conditions. The two plots in the graph below show the resulting drift for both low (yellow) and high (red) power settings. The results show a frequency stability of 3.8 and 5.0 ppm, respectively (both of which exceed Kenwood's 3 ppm spec). Note that the yellow trace in the graph below is taken with the TM-D700A operating at the same power level as the TH-F6 HT in the graph above. One would think the HT would be more prone to thermally induced drift due to its extremely small size (~ 5.5 cubic inches) and lack of heat-sink area, especially when compared to a much larger (~ 47 cubic inches), forced cooling fan equipped radio like the TM-D700A. The TM-D700A actually drifted more than twice as much during its first 1 minute transmission than the TH-F6 did over its entire hour-long test cycle, at the same power output levels!

Of course, these tests were being performed over a very limited temperature range (room temperature + temperature rise of radio under test), while Kenwood specifies frequency stability over a range of -10 to +50 degrees C. However, since the TM-D700A is already out of spec at room temperature, one can only imagine what the frequency error might be when subjected to the harsh, real world temperature extremes of an automobile interior!

Kenwood TM-D700A drift at room temperature

Next, I decided to test the TM-D700A's long-term frequency stability in receive mode. The radio was powered up at room temperature, and a short, low-power transmission was made for about 3 seconds, just long enough for the service monitor to obtain an accurate frequency error measurement. The radio was then left squelched in receive mode for 30 minutes. This cycle was repeated over a period of 10 hours. As can be seen in the graph below, the radio falls outside of Kenwood's published frequency stability spec after less than an hour in receive mode at room temperature!

Kenwood TM-D700A long term drift in receive mode

Finally, I decided to run one last test simulating a wider ambient temperature range. Lacking an environmental test chamber, I improvised by placing the TM-D700A's main unit inside my vehicle for 4-5 hours on a recent, particularly cold autumn night. The radio was then brought back to the test bench, powered up, and immediately tested. A non-contact infrared thermometer measured the case temperature to be 27 degrees F at the start of the 60 minute test cycle.

Kenwood TM-D700A drift from 27 degree cold-start

Since drift of this magnitude is virtually unheard of in modern synthesized radios (the last radio I owned that drifted this much said HEATHKIT on the front panel), I assumed there might be some skeptics in the audience. So I decided to videotape the service monitor display during the cold-soak test (above plot) and posted the digitized results here as proof of what I have presented.


QUICKIE DIAL-UP WMV FORMAT 320x240 time-lapse, no audio, 1 hour compressed into 44 seconds (205Kb file size) WINDOWS WMV FORMAT 320x240 video with audio commentary, 16 minutes long (7.98MB file size) QUICKTIME MOV FORMAT 320x240 video with audio commentary, 16 minutes long (10.1MB file size)

Obviously, there was something very wrong with this radio! Kenwood probably got a bad batch of some out-sourced component and didn't catch it in time before some defective radios made it out the door. The two TM-D700A's I tested both drifted excessively, and had serial numbers that were 45 units apart. This would lead me to speculate that there are probably at least 44 other defective radios floating around out there, (maybe more) either on the air or on dealer's shelves. The disturbing part of all this is that under normal use, I never would have noticed the problem. It wasn't until I happened to have the service monitor warming up to test another radio while transmitting on the TM-D700A that the defect became apparent! And how may hams have service monitors available to check their equipment?

In all fairness to Kenwood, they replied to my inquiry regarding the frequency drift problem as follows:

Dear Kenwood Customer: Specification for frequency drift in the TM-D700A: 3 PPM. This means 1.35 KHZ drift on UHF. The specifications are guarantee, if there is problem, Kenwood will be glad to evaluate it at the service center.

I will be shipping my TM-D700A back to Kenwood within the next few days, and will update this page once the radio comes back from repair. In the mean time, I would urge everyone who recently (October, 2003) purchased a Kenwood TM-D700A (especially units with serial numbers in the 505000XX block) to have the UHF transmitter frequency stability checked on a service monitor while the radio is still under warranty!