CIRCUITS

Essential for digital transmission is careful filtering to help keep the signal linear.Doing this at a low level is more manageable but is still lossy. Combined with the digital encoders tiny output level we need two stages to get enough drive needed by the PA. Here is a combination of filter and amplifier. Modified from a dual ERA5 board, this uses a more beefy output transistor to give up to a watt drive to the PA, (Be warned:800mW only required for a dual RA18H PA. A single RA18H PA will only need a quarter of this, they dont like it up 'em you know).

Click image for layout pdf

If L11 and L12 dont etch properly try a 2 turn coil on an 8mm former instead.

*VIDEO* *CIRCUITS*

Some useful video /atv circuits.More to follow soon

CCIR pre / de emphasis

I have found that the 300R gives rather peaky results in the Comtech tx and a 270R seems to give better results. The in/out 75 ohms are to remind you that the circuit must 'see' 75 ohms, so they may need to change dependant on the circuit loading and they can be series resistors instead of to ground as long as the filter looks into 75 ohms at both ends.This is essential --- any imbalance and the roll-off curve will be distorted and may lead to overshoot on hf. For a pcb layout to make your own board see projects



Low-distortion wideband video amp

A low cost 8 pin SMD video circuit which is happy with inputs from 0.25v pk to 1.5v pk. Using the preset shown the gain can be adjusted over a lmited range with little effect on bandwidth.The device is stable even at high gains. Using a dual supply no coupling capacitor is required, but shown here is a false 0v line from a single 12v supply.The input switch is optional,it is part of the repeater selection circuits.It is driven either +5 or 0v (e.g. TTL or a switch) Simply omit R8,Q1,R3,C5,R2 if not required.Click picture for a pdf copy. Don't be put off by the surface mount style, take a look at the Making PCBs the Easy Way
WORTHING MODULATOR IMPROVEMENTS
Despite a couple of improvements to the original design, these useful low-power 23cms transmitters still suffered from poor syncs, colour crushing and colour beat pattern and a non-standard pre-emphasis circuit. The design shown was intended to overcome these shortcomings.Q2(Tr6 in original circuit)must have an hfe of over 100 at 150mA which is beyond the BC557 capabilities this can cause crushing on peaks and can give chroma distortion. The pre-emphasis is nothing like the CCIR-405 especially after capacitors to' improve' the hf are added. An emitter-follower is added to allow a lower impedance feed to the audio/video balance preset enabling a notch filter at 6Mhz to be used to ensure that no harmonics can get through to the mixer and cause a beat pattern with the audio.The subcarrier is fed into R18 it mixes with the video on R17 and joins the modulator at point B in the original circuit at L3/R5 junction. C5 really should be a non-polarised capacitor, or perhaps two 220mfd back to back.R9 was dropped from the original 300 ohms as it seemed to cause some overshoot, probably due to the Q of the SMD version of L1. See PROJECTS page.


PA MODULATOR AMPLIFIER

Essential for digital transmission is careful filtering to help keep the signal linear.Doing this at a low level is more manageable but is still lossy. Combined with the digital encoders tiny output level we need two stages to get enough drive needed by the PA. Here is a combination of filter and amplifier. Modified from a dual ERA5 board, this uses a more beefy output transistor to give up to a watt drive to the PA, (Be warned:800mW only required for a dual RA18H PA. A single RA18H PA will only need a quarter of this, they dont like it up 'em you know). Click image for layout pdf If L11 and L12 dont etch properly try a 2 turn coil on an 8mm former instead.

Take a look at the PROJECTS page for more useful circuits...