7T9521 “Komukai40,”yet another 7MHz QRP Transmitter

Featuring class C SEPP output stage

7T9521 “Komukai40” with 7R3357R “Fujimino40” at Iwajuku, Midori-city (JCC1612).

15 April, 2006
Updated on 25 April, 2006


1. Introduction

In 2005, I had built a transmitter featuring pre-mix VFO unit using ceramic resonator VXO. Since the VFO stability was not enough for field operation and collector efficiency at the final stage was so poor, I decided to build another configuration.

The new transmitter, 7T9521 (the codename comes from "7MHz TX, 2SA950/2SC2120 for the final stage devices"), adopts varicap controlled VXO for stability and features class-C SEPP, which has been successfully experienced and implemented before, for high efficiency and low harmonic distortion.

7T9521 now has a nickname "Komukai40." The name comes from the place where Toshiba R&D Center locates.

2. Circuit description

7T9521 schematic
Fig.1 7T9521 schematic (click the figure to enlarge).

2.1 The VFO section

The VFO section consists of a varicap controlled Super VXO and a common-source tuned amplifier. A resistor parallel to L1 is for anti-parasitics and its value should be determined by cut & try basis.

The buffer output is followed by an LPF with 2nd harmonic trap.
Supply voltage for this section should be well stabilized. I used a 5[V] low drop regulator with LED level shifter to obtain 8.8[V] supply voltage.

It covers 7.001 to 7.009[MHz] and the 8[dBm] output is achieved.

2.2 The driver stage

The driver stage is a class AB tuned amplifier. The idle and operation currents are 11[mA] and 22[mA], respectively. Since the output impedance is estimated to 220[Ω] and the resonator QL becomes approximately 4.5, the driver resonator made of fixed value capacitor and inductor is sufficient for resonance frequency tolerance.

The keying is made at collector supply of this stage. To suppress key clicks, time constant elements (33[Ω] and 33[μF]) are inserted at supply line.

The output swing is 13[Vpp] for 220[Ω] load at 13.5[V] supply and it is enough to drive the SEPP power stage.

2.3 The power stage

Push-pull configuration is a good choice for power amplifier because its even order harmonics are smaller than the single amplifier case. The successful class C SEPP configuration (the design procedure is here) is employed again.

2SA950 and 2SC2120 are complementary pair made for 1[W] class audio amplifier, but also can be used in lower HF band. You can find similar devices for alternatives. For example, BD135 and BD136 are candidates for 2[W] or more: but it is not tested at my site :p

Harmonics are rejected by the 5th order LPF with transmission zero at 14[MHz].

2.4 Keying and break-in

A relay controlled semi break-in and the supply keying is employed.
The relay switches antenna connection and VFO supply line. Break-in delay can be adjusted. For full QSK application, the relay should be replaced by electronic switches.

2.5 Remote control

For transceive operation, some control connection with the receiver should be implemented. In this design, RX antenna, power supply, remote and the keying signals are provided for this purpose.

3. Implementation

The whole circuit shown in Fig.1 is implemented onto a 9572[mm] universal breadboard and the whole transmitter including 8(eight) AA batteries  are put into 15040100[mm] enclosure, as shown in Fig.2.
Note that the enclosure comes from 7T2120pp, and its breadboard is replaced by 7T9521.

Fig.2 7T9521 photographs (click picture to enlarge).
front-top view top view breadboard
(a) Overview (b) Top view (c) Breadboard

4. Some measurement results

4.1 Preliminary evaluations

4.1.1 Driver section

Preliminary test results for the driver circuit is shown in Fig.3.
As seen from the input-output response, the circuit does not act as a linear amplifier. Class A biasing is required for linearity.

The frequency response shows that fixed value components for resonator works well for this application.
Note that the driver also responses to 3.5[MHz] input signal. The circuit also acts as a frequnecy doubler.

Output swing response to the supply voltage shows very good linearity. It means that the circuit can be used for QRP AM modulation.

Fig.3 Driver under test (RL=220[Ω]).
driver under test driver frequency response driver frequency response driver supply voltage response
(a) Experimental driver. (b) Input-output response. (c) Frequency response. (d) Supply voltage response.

4.1.2 Driver and power amplifier section

The experimental board and its performances are shown in Fig.4.

The output measurements have been carried out by reading peak to peak voltage value from waveforms on the oscilloscope without LPF. Therefore, note that the output includes both fundamental and harmonics components and the collector efficiency calculation is based on total output power which includes harmonics components.

The input-output response shows that the circuit has 1[W] output capability for 8[dBm] input signal and 60[%] efficiency can be expected.
It also works on 9[V] supply, but it goes to QRPp mode :p.

Fig.4 Driver and PA sections under test.
driver and PA under test input-output response supply voltage response
(a) Experimental board. (b) Input-output response (c) Supply voltage response.

4.2 Product performances

4.2.1 VXO stability

Frequency stability is tested under open-air, room temperature condition. As seen from Fig.5, it is very sufficient for operation.

Fig.5 VXO stability from cold start.

4.2.2 Output performances

The output waveform is shown in Fig.6.  It is easy understand that 1[W] output is achieved without visible harmonics. Although the PA transistors are standard TO-92 package and no additional radiator is provided, it works well. In addition, QRPp transmission can be expected even if the supply voltage drops to 9[V].

Fig.6 Output performance(click picture or figure to enlarge).
(a) Waveform at 50[Ω] load. (b) Continuous operation. (c) Supply voltage response.

back to home