MKII FM ATV Receiver
In the passed it was not hard to find a FM TV receiver, in the form of a Satellite receiver. These days these older analog units are coming harder to find and if you did come across one they often needed to be modified. So I decided to come up with a design to work with the narrower form of FM TV modulation that is used for ATV. With the success of the older MKI design, it was time to move on with the improvements of technology.
Digital ATV is a lot of fun to experiment with as Ralph (ZL1TBG) and myself have found out over the last five years. But the down side to DATV is the set up cost that comes with this equipment. Where by the analog technology has done the opposite and it has come right down in cost. This why the Auckland ATV repeater will continue to provide on going access for those who wish to use there current analog equipment.
With the newer generation Comtech L-band tuner modules there is a lot more I can do in ATV receiver design. This idea come from a discussion with John (ZL1JD) in regards to the low cost modulator project. The thinking was that one would complement the other and provider away to view your own signal. So far this receiver has out perform any other receiver I currently own and It's a big improvement over the older MK1 design.
1/ Wide tuning range 800 to 2200 MHz
2/ Wide and narrow IF settings
3/ Variable low threshold demodulator
4/ Dual switchable IF sound demodulator, 5.5 + 5.742 MHz and 6 + 6.5 MHz
5/ Local oscillator off sets for different bands
6/ +/- video output
7/ 22kHz LNB drive (if required)
8/ Wide video bandwidth DC to 5.2 MHz (without threshold)
9/ 12 volts DC powered
10/ Designed for ATV applications (setup for 18 MHz bandwidth)
11/ Can driven via RS-232 for remote use (ATV repeater sites)
12/ Rotary encoder driven from front panel via the LCD display
RX Circuit description:
The L-band tuner takes the incoming RF and mixers it down to an IF of 480 MHz, with two IF filters that are switchable one is wide (27 MHz) and the other is narrow (18 MHz). This turner arranged around a Mitel PLL chip the SP5055. This PLL is driven with an I2C serial interface that is controlled via a PIC 16F876A Micro-controller. The FM demodulater runs at the IF frequency and uses PLL as the demodulater. In the loop filter of this PLL the time constant can be increased by varying the low threshold pot. The AGC line is connected to the A to D input of the Micro and is converted in software to a signal readout on the LCD display.
IC1 Is a NE555 timer oscillating at 22kHz, this drives two transistors in a Totem pole configuration. This feeds D1 and D2 these Diodes set out as a voltage doubler this part of the circuit provides the turning volts.
IC2 The incoming base-band signal is DC coupled into pin 1 of the NE592-N8. This is where the gain is controlled via TR3 (MPF102 JFET) across pins 2 and 7. Where by the bias is set from the Micro in form of PWM output. Pins 4and 5 are the deferential outputs, this feed RL1 for the positive and negative video modulation. The common pin is the pick off point for dual sound demodulator.
TR4 (BC327) is a DC level converter stage, followed the first sound trap. Next is the PAL video de-emphasis network, the output side feeds on to the second sound trap. TR5 and TR6 (BC327 & BC337) are a standard feedback pair The video is fed into the base of TR5 which then drives TR6 the signal then comes off the collector of TR6 providing the video output drive. TR7 is nothing more then a buffer stage in the form of an emitter follower, to drive the into the 75 ohm video output.
IC3 is the TDA9821 dual PLL sound demodulater. This takes the outgoing signal from the relay from the NE592 stage. TR11 and TR12 (2xMPF102 JFETs) are configed as switches to switch between the two sound IF frequencies (A & B). Where A is 5.5 + 5.742 MHz and B is 6 + 6.5 MHz. Through a set of four ceramic filters which then feeds the two inputs of IC3 pins 1 and 15. The audio gain has been set by the use of the two 1.2k resistors and the two 2.2 uF caps. The audio output pins 7 and 8 are connected to the 50uS sound de-emphasis networks.
IC4 is a TL072 op-amp used as a dual channel line amp. The pins 2 and 6 are the two inputs. The audio gain is set by VR3 and VR4. The output pins 1 and 7 are coupled to the outputs which then provide the two audio output channels.
IC5 is the PIC 16F876A Micro-controller that drives all parts of this receiver plus 2 x 16 LCD display, reads the push buttons and the rotary encoder from the display PCB. I have written the software and layed out menu in the same structure that Kith (ZL1BQE) did for the MKII ATV modulator. As for the documentation I will do at a later date and put it out as a PDF manual, as I have done for MKII ATV modulator.
IC6 the MAX232 RS-232 level converter. I have added this component to provide remote access to software settings, this could be from a PC or a repeater controller. For example changing receive frequency or even displaying the signal level.
REG1, 2 and 3 are the on board regulators REG1 is 9 volts supplying the video and sound stagers. REG2 and 3 are 5 volt regulators two drives the analog parts of the circuit where by three does the same for the digital side. The PCB layout has two ground planes one each part of this circuit. The digital ground is connected to analog ground via L3 100uH choke, there by redusing any unwanted noise between each part of the circuit.
TR10 (IFR9540N) this P-channel MOSFET switchers on and off the LNB volts. This N-channel device is bias in depletion mode, there for you need take the gate negative to turn off this is done via TR9 (BC548).
Performance and testing:
I have tested the receive performance down to -96 dBm this without any preamp. At this level with threshold I managed to resolve a P2 picture in the noise.
Signal to noise measurements were:
-96 dBm P2 set to 18MHz bandwidth with low threshold S/N 12dB
-85 dBm P5 set to 18MHz bandwidth with low threshold S/N 23dB
-82 dBm P5 set to 18MHz bandwidth without threshold S/N 26dB
-79 dBm P5 set to 27MHz bandwidth without threshold S/N 29dB
As you can tell using threshold will only provide an extra 3dB signal to noise performance. By doing this it will effect the output waveform at the higher video frequencies. To give some Idea how this compares to AM modulation an home aerial installer aims for a signal level of -54 dBm (55 dBuV) as a reference for the same P5 picture.
I have tested different bandwidth and compared the results on a video waveform monitor. There definitely a difference in video quality over arrange deviation settings. At 27 MHz the high end frequency components such as at the transition points were well defined. At 14 MHz the best way to describe the picture would be say it was VHS quality, it had the same look to it. Rounded off edges and general softer look to waveform. This is why we have come up a compromise of 18 MHz, to balance up quality against bandwidth. This area of signal to noise vs deviation is a sprite subject in it’s self, possibly for something a later date. The video bandwidth is DC to 5.2 MHz only limited by the sound trap at 6.25 MHz. DC response is ok but could be better, due to C18 and R34 in the circuit.
The operating AGC range is from -30 down to the noise floor about -108 dBm. I have added a table in software to display readings in dBm as well as in a bar graft format.
Audio bandwidth tested from 20 Hz to just above 20 kHz with a small amount of phase shift at the top end. The frequency response on the lower sub-carriers frequencies the roll off is about 15 kHz. The bandwidth of the two sets of filters are somewhat different and this was reflected in these resalts I got.
Other design problems:
The Miller effect and how it applies to signal transistors such as the BC327 and BC337. How do you work around these limitations hence my dilemma. The way I did this was to drive these transistors in low impedance configuration. Even so to get DC to 7 MHz flat response was somewhat pushing these device to their limits.
Decoupling or isolating between video stagers was another problem area. This took me a bit of time to work out the best way to do this. 0.1 uF capacitor on DC supply simply was not good enough. I found by adding a 10uH choke in the supply line between each stage started to make difference. So I ended up by using the 10uH choke with two capacitors one was 0.1uF and the other was 100uF this did the job.
There is very little to here, adjust VR1 with a frequency counter and set to 22kHz. Set VR2 to output of one volt peak to peak to set the 22kHz injection level. Check C11 and C13 with an Oscilloscope and adjust to minimise the sound sub-curriers levels from video output.
Updates for the MKII board layouts:
1/ Replace R23 with 270 ohms
2/ Replace R19 with 470 ohms
3/ Remove C31
4/ Add a link to ground under the ceramic filters
Software menu layout
From VFO display the left button goes into the menu as per the diagram. By repushing this button you are able to move through the various part of the menu. The rotary encoder changes the values as required. When storing the data the left button will exit without saving and the right button will rewrite the information to memory location selected.
When in VFO mode the steeping sizes are 100 kHz or you can jump to 10 MHz steeps by pushing the button in on the rotary encoder and holding in.
Right button from the VFO will take you into memory mode where by you can load data out of any one of 40 locations.