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My Review of PCS Electronics' MAX -1 (Ver 1.5) FM Transmitter
02nd May '99
My review of PCS Electronics' MAX-1 FM Exciter kit
The bottom line first :
Very interesting, technologically advanced kit. Refreshing choice of ATMEL Microcontroller and Philips PLL Chip rather than ubiquitous Motorola set. Higher 4-7 W output with on-board RF Amp makes it self sufficient for many more applications. Unique method of tuning final stage and in-built diagnostics makes it easy for beginners and professionals.
For Marko Seruga of PCS Electronics (a.k.a. Mighty), the MAX-1 is what he is playing around with when he is not programming IBM mainframes in his day job and not trekking up the mountains of his home country. So I was happy to note that after a few false starts in mid 1998, he has finally got his MAX-1 kit in good enough shape to send me a free review unit. He has also taken my suggestion to appoint a USA based distributor for his kit so that he can tap the major USA market. After a long delay from the date the kit was first announced at Marko's web site, the MAX-1 is now totally ready to market. It has definitely been worth the wait !!
Product reviews are a very interactive and cooperative process. I found that I began reviewing version 1.5 and as I sent review drafts across to the kit manufacturer, we are already on Version 1.7 !! Marko's been very cooperative and already made various changes since the version I first received. In order to show you how the product evolves step by step, I have posted my original review interlaced with details of the latest kit version. As the whole review process is by definition so interactive, I have also put in Marko's comments as a kind of dialog between Manufacturer and Reviewer.
Description of the kit's circuitry:
The brains behind the MAX-1 is the 89C2051 CMOS Microcontroller made by ATMEL. This is an advanced variation of the 8051, an 8 bit Microcontroller originally developed by Intel in 1980. The 8051 is the world's most popular Microcontroller family, made by many independent manufacturers (truly multi-sourced). About 200 million 8051s and variants are shipped each year for use in imbedded control applications !! Please see the 8051 FAQ for more info.
The ATMEL 89C2051 boasts of many features over the plain vanilla 8051 : 2KB of Flash Memory, Analog Comparator, 2 Timers, LED Driver. At just US$ 3 each, the 89C2051 does pack a pretty punch. (PDF Data Sheet by ATMEL)
Marko uses the 89C2051 µCPU for programming the PLL chip, Driving the LCD display, reading the frequency up/down switches and for tuning / diagnostics.
The PLL is based on a Philips TSA5511 PLL Synthesizer Chip. (PDF Data Sheet by Philips). This chip is a PLL Synth normally seen in TV and VCR Tuners as it works up till 1.3 GHz. It uses low power, has some bidirectional general purpose input/output ports and an in-built 2 bit Analog to Digital Convertor (ADC).
The Philips PLL makes an interesting combination with the ATMEL µCPU and the EEPROM used to store the last set frequency as all chips use the I2C bus for data transfer. This makes data transfer between the chips really easy.
The 89C2051 can run at any frequency from just about 0 Hz to 24MHz. The PLL Chip on the other hand needs a minimum reference frequency of 3.2 MHz (where it conveniently offers frequency steps of 50 KHz). Marko therefore decided to design an oscillator based on a 3.2 MHz Quartz crystal and a couple of digital gates. This provides the system clock for the 89C2051 as well as the reference frequency for the PLL chip.
The actual VCO is a Hartley Oscillator based on a BF981 dual gate mosfet. It uses a BB204 dual Varicap for the Audio Modulation and another BB204 for the Error Voltage. Varicaps are not linear in their Voltage/Capacitance function. In a broadband design, the PLL loop's Error Voltage normally is between 2-10 volts i.e. an 8 Volt spread. In case you used the same Varicap for both the Error voltage signals and the Audio Modulation, it would operate at different regions of its Voltage/Capacitance graph depending on the desired frequency of operation. This would make the transmitter's sensitivity to Audio signals ( and therefore modulation level for a given level of Audio signal ) vary at different operation frequencies. Most professional gear isolate this effect by using two different Varicaps.
The VCO is followed by a MMIC ( Monolithic Microwave IC)
stage. These little ICs made by HP and others offer 12 dB gain
from 0 Hz to over 1 GHz. They are unconditionally stable. All
they need is 4 external components and you have a low cost stage
that is stable, high bandwidth and high gain with 50 ohm input
and output . Having a stable and high gain stage after the VCO
allows one to design the VCO for very low output power. This
makes the VCO more stable and have less noise.
( Details of HP's MSA-0285 MMIC )
The MMIC feeds a BFR96 stage that develops enough power to feed the final RF stage built around a 2SC1971 RF Transistor that can output 4-6W up to 175MHz.
Output from the 2SC1971 transistor is passed on to the Impedance matching circuitry ( Two trimmer caps ) that have to be tuned as per your real-world antenna. A Low Pass Filter is built-in on the PCB, this being even more essential for broadband transmitters.
Innovative digital circuitry makes tuning easy:
Marko seems to be good at effectively utilising every little feature in the chips he has chosen. The ATMEL 89C2051 has an in-built Analog Comparator. Marko decided to using this feature to make an interesting circuitry to help tune the MAX-1.
The basic assumption is that one is transmitting the most power if the RF voltage at the output of the transmitter is the highest. This is not necessarily true as one can have a badly designed antenna, mistuned antenna, wrong impedance antenna, VSWR issues and all other types of complications. However, most kit manufacturers rectify a sample of the RF voltage at transmitter output and light up an LED in proportion to the rectified RF Voltage. The kit builder is told that the brighter the LED, the more power being pushed out. Marconi mentions while reviewing a Veronica 1W Tx that this is fairly close to the truth.
Marko rectifies a sample of RF voltage at transmitter output and uses the built-in Analog Comparator to compare that with another voltage. The output of the Comparator drives a small piezo-electric buzzer. The lower the pitch of the buzzer, the higher the RF voltage and therefore hopefully the RF power. Kit Builders have to put the MAX-1 into a "TUNE MODE" and then tweak the trimmer caps on the output transistor to get lowest pitch on the buzzer = maximum output. This makes tuning very easy for the novice.
My comments on the above system are -
A) Broadcast Warehouse have demonstrated a broadband Low pass filter + impedance matching circuit on their 1W no-tune PLL exciter. They loose only about 10% power across the whole FM band ( into 50 ohm resistive dummy load). Maybe Marko could have used a similar circuitry making the whole tuning experience unnecessary.
Flip side is that such LPF as used in the B/W PLL work best if your antenna is perfect. In case your antenna is not exactly 50 Ohms at the transmit frequency ( that's the normal real-world situation) , you will need to tweak the impedance match circuitry for optimum performance. Here Marko's circuitry is extremely useful.
B) How bright an LED is good enough, how low a buzzer pitch is OK ? Maybe Marko should use a reference signal on one leg of the Comparator and then program the LCD display to act as a digital voltmeter and show exact RF Voltage in Volts. You would then have a perfect idea of your output. Marko informs me that his next version will do exactly that. In the meanwhile, you can use your own digital voltmeter and read the output voltage on the sampling cap to get a measure of the output RF Voltage.
Or Marko could have used the same building blocks he has now and made his kit show VSWR by reading both forward and reflected voltages. Marko's site hints that the next version might have a VSWR meter built into the kit !!
C) Expanding on the idea , if Marko alters his circuit to always monitor RF output voltage rather than only in "TUNE MODE", he could shut down the final stage in case of too low or too high a voltage. This would protect against too much input DC voltage for last RF stage, horribly mismatched antenna or even protect the transmitter against a break in the Antenna cable.
( Marko : LEDs are not linear. They do not emit light in proportion to the current through them. On the other hand the buzzer on the MAX- 1 outputs a frequency in inverse proportion to the RF Voltage. This makes it easier to tune for maximum RF Voltage at transmitter output.
Besides, have you ever seen a poor novice trying to tune a transmitter while keeping an eye on the LED and another eye on the trimmer caps at the same time ? A transmitter with a buzzer is definitely easier to tune.)
Built-in diagnostic features :
Marko has not forgotten the extra features on the Philips PLL chip either. He has used the built-in ADC to read the PLL Error voltage. In the "TEST MODE", the ADC reads the PLL Error voltage and transmits it to the µCPU which shows the status on the LCD display using simple English. If the Error voltage is too high or too low, it means that the PLL will have trouble locking at the band extremes. The documentation mentions how you can alter the VCO's tuning coil to get correct locking across the band. As Marko pre-assembles the coils, one would not normally have a situation of too high/low error voltage. This feature is more useful if you are altering the circuit to work at different frequency bands.
The general purpose ports on the PLL chip are used to sense voltages at various points in the circuitry. In case of failure of the earlier RF stages, the LCD display would show an error status. This system is not totally fool-proof but forms the basis of Marko's future expansion plans.
My comments -
A) Reading the PLL Error voltage and displaying status is an innovative feature. But should you not shut down the final stages in case you detect an error ? The MAX-1does not have a shut-down feature yet.
B) Reading test voltages to detect error conditions is an excellent idea as it would immediately tell the user where to look for problems. But again you really need to shut down the transmitter when you detect an error.
This consists of 8 printed sides of A4. It is very informative but does point you to Marko's online instructions all the while, even for critical placement instructions for the RF transistors. A kit builder without an Internet connection ( if there is one left) would find it a bit difficult to make the kit based on the printed documentation alone.
The documentation does include the complete circuit diagram, component layout diagram, theory of operation, kit building instructions etc. However to find the specs for the kit, you need to visit the Internet again !! ( Marko : Not any more in Ver 1.7 !!)
The MAX-1 being in a state of flux and constant design upgrade, the documentation refers to transistors that are no longer there in Version 1.5 and a method to store Station ID which is a feature not implemented yet. ( Marko : I needed the µCPU's limited memory for other functional stuff so I took out the Station ID feature as that's only cosmetic)
A few sentences on procedure to tweak the output trimmers would have been appreciated.
I have always wondered why kit manufacturers do not include a
page with some simple antenna designs, that being the "now
what ?" step after the novice kit builder has finished his
kit. Marko does point you to web sites with Antenna designs.
( Marko : Done in Ver 1.7 !! )
Marko does intend to redo his documentation to make it more uptodate and complete.( Marko : Documentation is already totally re-done in Ver 1.7 !! )
I finished building the kit in 5 hours time. It is not too difficult to build as there is enough place on the PCB to work easily.
The PCB is a good quality double sided board with screen printing for component placement. The components are of good quality. Unfortunately the capacitors supplied have such faint markings on them that maybe half the time I took to make the kit was actually used up in identifying the capacitors. To make maters worse, Marko in his generosity sent me a few spare caps so I did not have the advantage of counting each type of cap to figure out which was which.
As the PCB has plated through holes, you need to be sure you place components correctly the first time around as extricating a soldered part is particularly difficult in PTH boards.
Marko supplies the PCB with the coils already formed out of Cu-Ag wire and with one leg already soldered in the right place on the PCB. Do remember to solder the other leg yourself else you can spend days wondering what went wrong. This is particularly useful as the Hartley Oscillator uses a tuning coil with a small tap on it for the feedback. This would have been somewhat difficult for the first time kit builder. I would think that having pre-formed and pre- soldered coils reduces the problems that novice builders have thereby reducing Marko's rate of support calls and product returns.
The PCB has on-board Audio input and RF Output connectors, making it easy for the novice to connect up and start transmitting right away.
The supplied Samsung LCD module is Huuuuge, that makes it easy to read across a room.
The MAX-1 shows great promise as it is well designed and has some interesting building blocks which will be Marko's starting points for further experimentation on the MAX-1 platform + the newer kits in the series. I look forward to Marko's further inputs to this hobby.
A) Place final stage 2SC1971 vertically upright. That allows better heat transfer and also allows you to use the corner hole on PCB while mounting the PCB. Currently the Heatsink partly covers the mounting hole.
(See pic of Heatsink)
B) Dont solder the LED onto the PCB. As current version does not display PLL lock status on the LCD, you might like to place LED on front panel near the LCD.
C) Before you solder in the onboard Audio in and RF out connectors, decide if it is better to place connector on chassis and run wires to the connectors.
D) Place a heatsink on the 7805 Voltage regulator as well, it does run a bit hot.
A) Marko has a complete circuit dia as part of his documentation. This includes all component values.
B) Marko's web site has a Spectral Output chart and full specifications.
C) Marko also sells power supplies and enclosures for his kits.
D) Marko sells his kit in many forms : Completely assembled; PCB and all parts; PCB and programmed µCPU. This gives you flexibility of choice based on your level of experience.
Wish list :
A) Shut down of final RF stages in case of PLL unlock and also while user adjusts the frequency.
B) Shut down of final RF stage in case of system errors
C) Readout of VSWR instead of RF Voltage
D) Documentation to describe how to reduce the RF power output to 100-1000mW range in case you want to use the MAX-1 as an Exciter to drive an RF Amp rather than as a full fledged Transmitter.
E) Optional onboard Pre-emphasis for those who will transmit in Mono using only the MAX-1
F) Automatic direct store of last set carrier frequency into EEPROM after user presses freq up/down buttons instead of needing a "STORE ALL" menu mode. ( Marko : Done in Ver 1.7 !! )
G) Display of PLL lock status on LCD panel instead of only the on-board LED.
H) Faster scan rate when setting Frequency up/down.
Wanna have your own page here, write
about your own experiences, own review, refute existing review,
add comments to existing review, publish your circuits ?
review of the Broadcast Warehouse 1W PLL Exciter with LCD 08 Aug 1998
My review of the WaveMach FMS2 Stereo PLL Exciter 20 Dec 1998
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