Builder's Alert #11 

 K2 Application Note: Curing the ~7185 kHz "warble"
5 May 2000 

Applicability: K2s shipped prior to April 6, 2000. (The K2 manual instructions will be revised for all later shipments.)

 

Background:
Some K2 owners have noticed a peculiar "warble" quality to the received signals around 7185 kHz on the 40m band. Signals received close to this frequency may sound as though they are "passing through water", or similar to signals that are received under conditions of multipath or polar absorption. It is most noticeable on SW broadcast stations, that are using amplitude modulation (AM). In addition, other stations listening to the K2's transmitted signal may detect this warble quality.


Someone coined this behavior as the "Weird at One Frequency" or WAOF problem. This document provides a method to correct this. Technical details are provided at the end for those who are interested. We will change the manual for the K2 shortly to include these changes to the construction steps.

(Note: The VFO lockup present when the K2 is powered ON with a display frequency near 7185 kHz is a separate but related issue, and will be corrected in the next firmware release. It can be worked around by moving the VFO +/- 10 kHz and then moving to another band and returning.)


Changes Needed:

  1. Locate D16 and C70 at the front left corner of the RF PCB. 

  2. D16 needs to be mounted flush to the circuit board. By heating both leads of D16 together with the side of the soldering iron tip, you can press D16 down against the PCB easily and with minimal heat. This is preferable to removing D16 and possibly damaging this component. 

  3. Check for solder bridges between the pads of D16 and clean as necessary. D17 does not seem to be involved in this problem, so it may be left alone.

  4. C70 needs to have short leads and should be positioned close to D22. If necessary, remove C70 and scrape some of the ceramic coating from the leads of the cap so it can be installed very close to the PCB. Once C70 is mounted, bend it over slightly so that it just touches D22. 

  5. In some K2s (especially Field Test units), it may be be necessary to add a 10pF cap across U4 pins 1 & 2, using *very* short leads.

 

This procedure has worked on every K2 serviced to date.


Technical Details (optional):
Capacitor C70 has the full RF voltage of the VFO tank circuit on it. As a result, it emits some RF that is easily absorbed by D16. Since D16 is in the gate circuit of the 12.096 MHz PLL Reference Oscillator, it is susceptible to RF feedback from C70. The result is that there is an RF feedback path from the tank circuit of the VFO into the PLL Reference Oscillator at Q19. Q19's output is one of two important signal inputs into the MC145170 Phase Locked Loop (the other input is F_in from the VFO's output), so any phase distortion or undesired feedback can affect the PLL's function. Short leads in this area help insure circuit stability.

PLL chip U4's OSC_in and F_in inputs are at equal frequencies (approximately 12.096 MHz) when the K2 is tuned to a 5 kHz wide window around 7185 kHz, plus or minus small differences in displayed frequency because of BFO and PLL Ref. Osc. variations. These signal's phase relationships are compared in the PLL, and the output is fed into the PLL loop filter at U6B. This IC and its surrounding circuitry filter the pulses from PLL chip U4, and provide the tuning voltage to the varactor diodes in the VFO (actually a VCO, or Voltage Controlled Oscillator). 

Any noise that is present on U6B's output modulates the VFO frequency by a small amount. This is heard in the VFO's output as a warble in what should be normally be a steady signal. Once the VFO is tuned outside of this 5 kHz window, the PLL phase detector functions normally. If you have a wide bandwidth oscilloscope, you can monitor the waveform at U4's PD_out pin 13, and probably see a "fuzziness" on the trailing edge of the complex waveform until the corrective procedures described above are completed. 

Interested readers can obtain more information on the MC145170 PLL synth IC from one of several Motorola applications notes. 

Try:    http://www.mot-sps.com/books/dl110/pdf/mc145170-2rev4a.pdf 

The MC145170 has 4,800 transistors internally.

Be sure to check the Elecraft Builder's Resource Page often for technical updates, and other important information:   http://www.elecraft.com/Apps/K2info.html 


"It's Elecraft's goal to continuously improve our kits and to communicate circuit changes to our customers as quickly as possible. "