RF signal source. Oscillator prototype build & test

I am in the process of building an RF Signal Source as per EMRFD Fig 2.27. This is a Hartley design and incorporates 2 oscillators. The first covering a nominal 2-10MHz and the second covering a nominal 10-45MHz.

The Hartley topology is chosen for two reasons. a) The capacitor in the resonator can be fully variable and thus allows a wide range of frequency coverage unlike the Colpitts where, by definition, the feedback capacitors are fixed. b) The oscillator exhibits a reasonably flat output power over a wide frequency range.

Having built the chassis, installed the variable capacitors and cut the panels for the enclosure, I was now in a position to build a rough prototype of one of the oscillators. I chose the lower frequency oscillator. This took me a few hours to build. Since I don't have any of the specified 2N4416 JFETS, I decided to start with an MPF102. I selected sample 2 (Ref JFET experiments blog)  since it had a lower pinch-off voltage and higher Idss than sample 1, although not as good in performance as the tested J310. I decided to start with just the 400pf variable capacitor. This should allow easier troubleshooting since there are fewer parts. (refer to the schematic attached).

The oscillator powered up first time with no issues. The oscillator turn on voltage Vdd was 1.96V. Maximum output was obtained with a Vdd of 7.58V. Thus there was an excess of current flowing in the drain as Vdd was increased to 12V. The scope output waveform 'looks' clean. I have no way of measuring the harmonic distortion. Could increasing the rail voltage beyond 7.58V result in an increase of harmonic distortion?

The 3 turn link output was first terminated in a 50 ohm feed thru connector which was connected to the scope channel 1. This channel is also fed to the frequency counter. Then the output was connected to the Power Meter.

The oscillator is reasonably stable and certainly acceptable for general measurements and can be improved upon with more careful construction. Measuring at a frequency of 7030.44kHz and starting at 08:30 am the oscillator drifted upwards to 7030.46KHz after 23 minutes (10Hz). The shack door was closed. No thermometer is available. Then with the shack door opened and after another 10 minutes the oscillator was at 7030.73KHz. It then started to rain which presumably lowered the temperature the frequency measured was 7031.02KHz (562Hz). On blowing on the parts with a straw the indications were that the 10pf capacitor was the biggest drift contributor. As the temperature decreases the oscillator frequency increases.  Neither the JFET or the inductor seemed to be significant contributors. No vibration tests were carried out, however it was easy to tune the large capacitor to within 1KHz of the desired frequency. More precise tests needed once the buffer is built.

An error was discovered in the design. The Large 400pF variable capacitor wire runs parallel to the 30pf bandspread capacitor for about 4 inches. Although this capacitor was not connected it changed the oscillator frequency when it's capacitance was varied. Action here is to re-route the large capacitor wire in a different direction.

Tests with the Power meter showed an output variation of just over 1dB from an Fmin output of -1.33dBm at 2810KHz to -0.27dBm (frequency not measured but around 6000KHz). The output power at Fmax of 13663KHz = -0.75dBm. These measurements are in line with  those stated in EMRFD and very satisfactory.

Scope measurements
Fmax = 13663KHz (this frequency will decrease as the bandspread capacitors are added)
Fmin = 2810KHz (This frequency will increase as the bandspread capacitors are added)
Fmin Vpk-pk = 6div*0.1 = 0.6V (~0.5dBm)
Fmax Vp-p = 5.5div*0.1 = 0.55V
F(7175KHz) = 0.6V
Power Meter measurements
Fmin = DVM = 1770mV, thus Power = -86.29+(0.048*1770) = -1.33dBm
Fmax = DVM = 1782mV thus Power = -86.29+(0.048*1782) = -0.75dBm
Fpk = DVM = 1792mV thus Power = -86.29+(0.048*1792) = -0.27dBm

Next steps
Reroute the capacitor wiring.
Try different 10pf capacitors to try to improve thermal stability
Try a J310 JFET.

Prototype Hartley oscillator 2.8MHz - 13.7MHz

Pic showing the large variable and bandspread variable and oscillator circuit with terminations.

Oscillator set at 7030.64KHz with scope waveform showing no 'visible distortion'. Harmonic level not measured. The feedthrough 50 Ohm terminator on channel 1 can be seen. 

W1FB Direct Conversion Receiver experiments 1

As a part of my project to build a field radio NVIS system, the scope of which I have described in previous blogs, I have now built a first version of the W1FB Direct Conversion Receiver. This venerable design is fully described in W1FB's QRP Design Notebook so I won't repeat it here. I built my version using Ugly style construction and specifically Manhattan style. This being a pleasure and possible with only a hacksaw to cut the PCB and a pair of side cutters to cut out the 'islands'. See a pic below.

My challenge was that I did not have the same components for the Colpitts/Clapp design Oscillator. So I set about seeing what I could do with the components on hand. Certainly I did not have a T-50-6 material toroid. I used a T-50-2 instead. This core is not designed for temperature stability use but I had no choice. The trimmers I have are also probably not suitable and are SMD type trimmers. In addition the capacitors are all ClassII X7RO types. Oh well I will learn something for sure.

The calculations I did are as per the included paper below. I found a calculator online that helped greatly and eased the tedious maths a bit. I also measured the actual inductor and capacitor values using my L/C bridge. Once I had the oscillator built I then used my MFJ259 to verify that indeed the circuit was resonating in the 40meter band. My half turn trimmer covered from 6.6MHz to 7.2Mhz. This was going to be a challenge to tune!

I started by building the LM386 based audio amp. Verified that it was working by simply touching the input with my finger and hearing hum in the earphones. I then added the Audio preamp and conducted the same test. I certainly could hear added gain and louder hum in the earphones. The components were all as per W1FB's design except the speaker coupling capacitor which is a 100uF unit. I supposed that the low frequency response is less as a result of this.

I also have not yet built the audio filter, figuring that the mission was to just get a platform design working.

My design has a smaller inductor of 1.38uH and is inherently less stable.

When I first turned on the rig I heard nothing. I could peak the noise on the tuned circuit so I figured that it was working. I then connected my 2meter J pole to the input and could hear a few very low level signals. I then transferred to the freezing patio outside where my 40m efhw is located. Man was it cold and now 2am in the morning. But I had to see if I could hear anything.

I connected my efhw and put in the earplugs. On turning on the rig I was greeted with a very loud blast of sound! I could not believe it! On turning the volume down to a minimum and tuning the oscillator with a tuning stick I was able to copy loud CW and SSB signals coming in from Europe and the US. Excellent! Over the next few days I tested the rig at all times of the day. During the mornings there is loud local broadcast interference. During the afternoons the rig works fine except that the bandwidth is very wide (not surprisingly). The dynamic range of the NE602 seems very good. Tuned to the CW section of the band I can hear local SSB up frequency. So there is plenty of room for improvement. Adding and audio filter will greatly improve daytime operation. I am less sure about how to cure the BCI however without adding excessive filtering on the front-end. Microphonics seem under control and also there is no hum. all good. There are 4 areas for improvement in the following order of priority.

1. Improve the VFO. First arrange the capacitors to allow narrow band coverage with the trimmer. This is going to be tough since only a few picofarads are needed to tune the 40Khz CW band. Second I can do some things to the physical construction to improve stability. Mainly securing the inductor and windings better. Having said this I am very happy with the short stability as is. Once the rig is tuned on frequency there is no drift discernable during the period of  a typical QSO.
2. Improve the selectivity by adding an active audio filter. This filter should be optimized for CW reception and should eliminate the higher band ssb interference from loud local stations.
3. Improve the front-end selectivity by perhaps adding a double tuned front-end parallel circuit. This should help eliminate the SWL broadcast interference when the band goes long in the evening.
4. Put in a BCI filter and shield the receiver to eliminate the very strong BCI that swamps the radio in the mornings. 

W1FB DC Receiver

ZS6RSH version 5/24/2013