A VHF VCO for 100...200 MHz
One favorite project I have in mind is a homebrew spectrum analyzer. The homebrew spectrum analyzer
project of Wes Hayward, W7ZOI described in an older QST issue was the inspiration for me. Allthough nowadays
VCO hybrids are available for a all frequency ranges from HF to SHF I wanted to assemble an discrete
one as a first lesson. See the attached picture for the assembly. I used ugly construction style with
SMD's and "conservative" piece parts, just what was available in the shack for a small set up.
Here is the
.The Low Pass Filter was still not assembled when the photo was taken.
30 MHz Frequncy Counter with 2...255 prescaling availability
This frequency counter was my first microprocessor project. The only previous contact with microprocessors was
during my time of study. So, after some lessons of learning by doing I was prepared to start with this
project using a ATMEL ATMEGA8. Against most microcontrolled counters I don not use the processor for
counting as for data processing. The counting unit is assembled by real hardware consisting of 8 BCD
counters 74HC160. The counting result is transferred into a line of 4 74HC165 8-bit parallel to serial
shift registers. The register contents are shifted serial into the processor memory for further
handling. Allthough this is a high hardware effort it has a special advantage against circuits using
the processor itself as the counting element. My counter has a resolution of 1Hz at a maximum input
frequency defined by the used counter IC's. In case of the 74HC160 I used, the counter works up to
30 MHz with a resolution of 1 Hz. Using the processor as the counting element the max. input frequency
is mostly in the 50kHz-5Mhz region (dependent of the processor and the internal counter register).
For higher frequencies you have to use prescalers at a very early stage leads to a limited resolution
at a early stage of some MHz. With my counter project I tried to get better resolution when using
prescalers esspecially for UHF/SHF frequencies. For the display I (wanted) to use 7-segment LED's
rather than a LCD. As display driver I used the Intersil ICM7228 8-digit, microprocessor compatible
LED display decoder driver. The prescaling factor of the counter is set by a 8-bit switch array on
the front panel. The setting of the switches is taken over by a 5th 74HC165 8-bit par-to-ser.
register. This 5th shift register is attached to the chain of the 4 shift registers which were read
out by the processor. So, the prescaling factor setting is read out with each counter result read
out. The microcontroller controls the whole process. This is the GATE timing, result shifting into
memory, frequency calculation and display in dependence of the prescaling factor. The whole circuit
is built on a experimenters board. To bring the input signal to a right level of the digitals I used
a NE592 video amplifier circuit on a seperate board. See the pictures for assembly details. You are
welcome to contact me for any detailed requests.