Call Search

New to Ham Radio?
My Profile

Friends Remembered
Survey Question

DX Cluster Spots

Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
Author Topic: IC-765 FSK RTTY splatter  (Read 3157 times)

Posts: 11

« on: December 02, 2007, 03:24:43 PM »

I had a report today that my RTTY signal was splattering and had no middle.  I am using FSK and NOT AFSK.  I see an over full scale indication of ALC when transmitting in the RTTY-FSK mode. Is this normal on this radio?  No mention of it in the operation manual.  No tx audio if I switch to SSB when N1MM is txing FSK.  Is ALC indication normal when in FSK?  

Posts: 11

« Reply #1 on: December 04, 2007, 08:57:00 AM »

I think I have figured out what is happening to my RTTY signal. The IC-765 in FSK mode, is direct FSK. The TTL line modulates the VCO. No AFSK generator. So, I closed the telnet port and my signal cleared up. My computer is running too slow (500mhz w/ 100mhs Bus) to support all of the operations. I found a faster CPU (800 mhz) on E-bay for $3.00. It's worth a try.

Posts: 1113

« Reply #2 on: December 09, 2007, 09:46:45 AM »

True FSK has nothing to do with "modulating" a VCO.

True FSK, "Frequency Shift Keying", actually "shifts"
the actually RF carrier frequency between a MARK and
a SPACE. This is usually accomplished by "changing"
the RF oscillator's frequency through the use of a
capacitor. By adding a capacitor across an oscillator's
coil, you change the RF frequency of the oscillator.
In most cases this is accomplished by a diode switch.
A bias voltage is applied to the diode which switches
in the capacitor causing a change in the RF oscillators
frequency. Removing the bias opens the diode switch,
removing the capacitor causing the RF oscillator to
return to it's original RF frequency. This creates
the "shifting" MARK and SPACE output frequencies.

AFSK, "Audio Frequency Shift Keying", however, uses
2 separate audio tones to produce MARK (2125hz) and
SPACE (2295Hz) "shift". The same basic concept
occurs except the "shift" takes place in an "audio
oscillator". The audio oscillator's output is fed to
the transmitter's modulator stages, usually at the
input of the second stage of the speech amplifier to
prevent overdriving the speech amplifier, and the
resulting RF output signal "appears" to be "shifting".
The AFSK tone oscillator is usually an external
device such as a sound card or a terminal unit (TU).

The RTTY "shift" standard is 170Hz whether it's true
FSK or AFSK. The older less used 850Hz shift was
changed to 170Hz to conserver bandwidth and lessen
garbled text due to fading.

I'm sure a lot of old timers remember shifting
the VFO's to create RTTY. I did it with a
Hallicrafters HT-32, a capacitor, a "D" cell battery,
and a spare set of contacts on the Model 28
KSR teletype I had. I didn't need a diode since
I used the spare set of keyboard contacts on
the Model 28 KSR teletype I had. It took some
adjusting but once it was set properly, it
worked very well! The "D" cell battery would
last a year or more. I still prefer true FSK over
AFSK anyday!!


Posts: 15044

« Reply #3 on: December 09, 2007, 04:20:15 PM »

As I recall, the 765 operates by switching in an extra variable capacitor in one of the injection oscillators to create the FSK shift. Part of the calibration procedure is to adjust this capacitor to obtain the proper 170Hz difference between mark and space.

I expect what is happening is that your computer is putting out some high speed "garbage" on the serial line that is causing the 765 to shift between mark and space at a much higher rate than the normal 45 baud and that is causing a very wide and distorted signal. The 765 doesn't know what code or data rate is coming into it. It simply sends mark or space frequencies depending on whether the FSK input is open or grounded. If you feed it 9600 baud ASCII it will attempt the transmit 9600 baud ASCII. The higher the baud rate the wider the signal and the wider the signal the more distortion caused by the filters and other circuits which were designed with low speed baudot in mind.

73, Bob

Bob  AA4PB
Garrisonville, VA

Posts: 55

« Reply #4 on: January 12, 2008, 08:06:38 AM »

I upgraded to a 800 mhz cpu and it cleared up the splatter.  Computer was running too slow......
Pages: [1]   Go Up
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!