An Auto switching Oscillator for the DSP10 Transceiver

Dave Robinson, WW2R, G4FRE

Introduction

Recently I have started to use the W7PUA DSP10 transceiver (1) on microwave EME. It quickly became apparent that the onboard crystal oscillator circuit is not good enough for digital modes such as JT65C (2) with respect to temperature stability. The DSP10 does have provision for connecting an external 10MHz reference (3, 4, 5) but changing between the two sources is problematic. The appropriate connector, P106 is located under the DSP unit with little clearance, The DSP module has to be taken out to change the link. Initially thoughts turned to temperature compensating the onboard source, but it was quickly realised to be an unviable option. The usual 10MHz precision oscillators (HP10811) were too large to even get in the box. At this point some compact Toyocom units were advertised by DEMI and one was obtained.  All that was needed was the circuitry that would automatically switch this off and route an external 10MHz source to the DSP10. This writeup describes such a unit

Circuit Description

The circuit is shown in Figure 1. Q1 buffers and splits the input signal. One output signal drives Q5B gate that buffers the off board 10MHz source. The other output of Q1drives a rectifier (D1), the output is used to drive switch Q2. Q3 and Q4 buffer and invert this signal to switch the supply voltage to the Toyocom Oscillator Q5A buffers the 10MHz output from the onboard oscillator. Q5C switches between the two oscillators. The output of Q5C goes to the 10MHz input of the DSP10 board, J106, at TTL levels as required by LMX1501A (U104) on the DSP10 board. Although the output is rich in harmonics it was not found necessary to use a low pass filter before my LMX1501A.

Construction

In view of the space restrictions in the DSP10 unit, a PCB was needed for the unit. The components would have to be SMT, using 0805 size chip passive components. To achieve the space saving, plated through holes and very small tracks were needed. Production of such PCBs were beyond my production capabilities, so boards were produced by Expresspcb, using their free software (6). Q3 contains a complimentary pair of FETS. Two wire links are needed on the board. A negative voltage applied to the oscillators tune  can be used to set the frequency of the oscillator. Alternatively the tune pin can be earthed and the frequency setting done with the adjustment on the oscillator. Figure 2 shows the component overlay. Figure 3 shows the top view of the unit. Figure 4 shows the unit installed in the DSP10 Transceiver.

Figure1. Circuit Diagram

Table 1. Component listing

 

Figure 2. Component layout

Figure 3.Top view of unit

Figure 4. Unit installed in DSP10

 

Conclusions

DEMI is now out of the original oscillators but a lot of them were sold. Similar units have been seen on auction sites. The board can be used with other oscillators with different pinouts.

References

 

1. http://www.proaxis.com/~boblark/dsp10.htm
2. http://pulsar.princeton.edu/~joe/K1JT/
3. http://www.realhamradio.com/GPS_Frequency_Standard.htm
4. http://www.rt66.com/~shera/index_fs.htm
5. http://mysite.verizon.net/n1jez/osc/page5.html
6. www.expresspcb.com