Specifications

ManualsBrandsSILTRONIX ManualsConsumer electronics1011B

Page 14

CIRCUIT THEORY

GENERAL DISCUSSION

The Siltronix 1011D transceiver provides single sideband, suppressed carrier transceive operation, and generates the single sideband

signal by means of a crystal lattice filter. To permit a logical discussion of this mode of operation, certain definitions are necessary.

In a normal AM signal (double sideband with carrier) a radio frequency signal is modulated with an audio frequency signal. This is

considered by many to be merely a case of varying the amplitude of the carrier at an audio rate. In fact, however, there are actually

sideband frequencies generated, which are the result of mixing the RF and the AF signals. These sidebands are the sum of, and the

difference between, the two heterodyned signals. In the detection of this conventional AM signal, the two sidebands are mixed with

the carrier to recover and reproduce the audio intelligence. This is an inefficient means of transmission because only 25 percent of the

transmitted power is used to transmit intelligence. There are other attendant drawbacks also. The bandwidth of AM voice

transmission is approximately 6 KHz, while the actual demodulated audio is only approximately 3 KHz. The result is inefficient use

of the frequency band, and over half of the allotted band is unusable due to heterodynes, interference, and congestion.

In the single sideband, suppressed carrier mode of transmission, only one of the sideband signals is transmitted. The other sideband

and the carrier are suppressed to negligible level. In addition to increasing the transmission efficiency by a factor of four, single

sideband effectively doubles the number of stations or channels which can be used in a given band of frequencies.

It should be remembered that in the single sideband, suppressed carrier mode of transmitting, the unwanted sideband and carrier are

only suppressed, not entirely eliminated. Thus, with a transmitted signal from a transmitter with a 50 db sideband suppression, the

unwanted sideband will be present and will be transmitted, but its level will be 50 db below the wanted sideband. When this signal is

received at a level of 20 db over S9, the unwanted sideband will be present at a level of approximately S5. The same is true of carrier

suppression. With carrier suppression of 60 db, and a signal level of 20 db over S9, carrier will be present at a level of approximately

S3 to S4.

For the following discussion refer to the schematic diagram and to Figures 3, 4, and 5.

SIGNAL GENERATION

When the push-to-talk switch on the microphone is depressed, the transmitter portion of the transceiver is activated, and it generates a

single sideband, suppressed carrier signal in the following manner. Carrier is generated by Carrier Oscillator Q3, connected as a

Pierce oscillator with the crystal operating in parallel resonance. This stage operates in both the transmit and receive modes. When

transmitting, the RF output of the oscillator is injected into the control grid of the Balanced Modulator, V13. This balanced modulator

is a beam deflection tube, and operates similar to a cathode ray tube in that the electron beam from the cathode is deflected to one

output plate or the other by the charge appearing on the deflection plates. The carrier signal applied to the control grid of the balanced

modulator appears on both plates of the output. The two plates are connected to Transformer T1301. The deflection plate DC

voltages are adjusted by means of the carrier balance control, R1305, so that the RF signals being applied to the output plates will

cancel each other, and the output from T1301 will be zero. Audio

signals from the Microphone Amplifier, Vii are applied as a

modulating voltage to one deflection plate and the two sidebands resulting from the sum and difference frequencies of the audio and

carrier signals appear in the output of transformer T1301. Carrier suppression is approximately 60 db down. The Carrier Insertion

control limits the carrier level that can be inserted in AM and thus protects the final amplifier from being overdriven.

The double sideband, suppressed carrier signal is then coupled from the secondary winding of T1301 to the crystal filter, which

suppresses the lower sideband, and permits only the upper sideband to be applied to the First IF Amplifier, V7. The carrier frequency

is generated at approximately 5500.0 KHz, when the unit is in the upper sideband mode (USB). With the lower LSB sideband crystal

the carrier crystal frequency will be 5504.6 KHz, and this positions the double sideband signal on the other side of the filter response

curve, attenuating the upper sideband by at least 50 db.

Q1, the VFO 2N706 Oscillator, operates in the common base configuration as a Colpitts oscillator. Q2, the buffer, is used for

isolation. The extremely good regulation achieved through using the Zener diode regulator, D1712, across the bias supply voltage,

also contributes to the stability.

The VFO, in the Model 1011D, exhibits extremely good stability after the initial warm-up period. Drift from a cold start will be less

than 2 KHz during the first hour. After the initial warm-up period drift will be negligible.

The single sideband, suppressed carrier signal from the First IF Amplifier is fed to the Transmit Mixer, V2, where it is heterodyned

with the VFO signal. The resultant signal at the desired transmit frequency is amplified by the Driver, V3, and the Power Amplifier,

V4.