SL623

Supply current

Input impedance

SSB audio output

AM audio output

AGC range (Note 1)

Conditions

ELECTRICAL CHARACTERISTICS

Test conditions (unless otherwise stated)

|

Ambient temperature: -30

°C to +85°C

Test circuit as Fig.2

Characteristics

ABSOLUTE MAXIMUM RATINGS

Supply voltage: 12V

Storage temperature: -55

°C to +125°C

Max.

Min.

22

43

9

800

30

55

Typ.

11

47

67

6

Value

mA

Ω

mVrms

mVrms

dB

Units

No signal, Pin 4 open

Pins 6, 9

Signal input 20mVrms @ 1.78

MHz. Ref. signal input 100mV

@ 1.750MHz

Signal input 125mVrms @ 1.75MHz

modulated to 80% at 1KHz

Initial signal input 125mVrms at 1.75MHz

modulated to 80% at 1KHz. Output set to

2.0V with 10K

Ω potentiometer between

Pins 2 & 5

NOTES

1.The AGC range is the change in input level to increase AGC output voltage from 2.0V to 4.6V

APPLICATION NOTES

AGC Generator

Pin 3, the AGC amplifier phase correction point should be

decoupled to ground by a 1 microfarad capacitor (C4),

keeping leads as short as possible The value of C4 is quite

critical, and should not be altered: if it is increased the

increased phase shift in the AGC loop may cause the receiver

to become unstable at LF and if it is reduced the modulation

level of the incoming signal will be reduced by fast-acting

AGC.

The AGC output (Pin 4) will drive at least two SL610/11/ 12

amplifiers.

The SL623AGC output is an emitter follower

similar to that of the SL621C. Hence the outputs of the two

devices may be connected in parallel when constructing AM/

SSB systems.

Less signal is needed to drive the SSB demodulator than the

AM detector. In a combined AM/SSB system, therefore, the

signal will automatically produce an SSB AGC voltage via the

SL621C as long as a carrier (BFO) is present at the input to the

SSB demodulator of the SL623C. The AGC generator of the

SL623 will not contribute in such a configuration.

For AM operation the BFO must be disconnected from the

carrier input of the SSB demodulator. In the absence of an

input signal, the SL621C will then return to its quiescent state.

To switch over a receiver using the SL623C from SSB to AM

operation it is therefore necessary to turn off the BFO and

transfer the audio pick-off from the SSB to the AM detector.

Neglecting to disconnect the SSB carrier input during AM

operation can result in heterodyning due to pick-up of carrier

on the input signal In some sets different filters are used for AM

and SSB: these will also need to be switched.

The 10 kilohm gain-setting preset potentiometer is adjusted

so that a DC output of 2 volts is achieved for an input of 125mV

rms There will then be full AGC output from the SL623C for a

4dB increase in input. A fixed resistor of 15kilohms can often

be used instead of the potentiometer.

SSB Demodulator

The carrier input is applied to Pin 6, via a low-leakage

capacitor It should have an amplitude of about 100mV rms and

low second harmonic content to avoid disturbing the DC level

at the detector output.

Pin 8 is the SSB output and should be decoupled at RF by

a 0.01 microfarad capacitor. The output impedance of the

detector is 3 kilohm and the terminal is at a potential of about

+2V which may be used to bias an eminer follower if a lower

output impedance is required. The input to the audio stage of

a receiver using an SL623C should be switched between the

AM and the SSB outputs — no anempt should be made to mix

them. Since the SL621C is normally used in circumstances

where low-level audio is obtained from the detector, the

relatively high SSB audio output of the SL623C must be

attenuated before being applied to the SL621C. This is most

easily done by connecting the SL623C to the SL621C via a 2

kilohm resistor in series with a 0.5 microfarad capacitor.

Input Conditions

The input impedance is about 800 ohms in parallel with 5pF

Connection must be made to the input via a capacitor to

preserve the DC bias. An input of about 125mV rms is required

for satisfactory carrier AGC performance and 20mV rms for

SSB detection. Normally, the AGC will cope with this variation

but in an extreme case a receiver using an SL623C and having

the same gain to the detector in both AM and SSB modes will

be some 10dB less sensitive to AM.