More URX-1 Utility Reciever Info

Major Features
General Description
Technical Description
RS232 Details
More Info

Major Features

Single-band VHF/UHF NBFM receiver
10 MHz single band span x 1.0 KHz steps
Band range : 100-1000 MHz
Speaker output : 1 Watt
RSSI analog output ( available )
100% remote control via RS232
DC 11-18V, 30 mA ( squelch closed )
1.75 x 3.78 x 0.35, 0.2 ounces


Sensitivity better than 1.0 uV for 20 dB quieting, with unmodulated carrier
Tuning range 10 MHz x 1.0 KHz steps
( user specifies 10 MHz band, between 100 and 1000 MHz )
Bandwidth 15 KHz standard
Audio Output 1.0 Watt into 8 ohms
RS232 Profile 4800 baud 8N1
RSSI Range -120 dBm to -60 dBm, analog level
DC Power 11-18 VDC, 30 mA idle, squelch closed
Size / Weight 1.75 x 3.78 x 0.35, 0.2 ounces


Connections to / from the URX-1 are shown in the diagram below. The host computer is only required to change a parameter or record information from the URX-1. ( pretty simple... )

General Description

The URX-1 Utility Receiver is sold as an assembled / tested PC board, with no enclosure or connectors. It is a single-band VHF/UHF FM reciever with RS232 control of frequency, audio and squelch levels. Control settings are saved in non-volatile memory, and restored whenever DC power is applied. ( CPU coldboot )

Intended for use as a "companion" reciever for the DF equipment on this website, it can also serve as a general-purpose FM reciever, with speaker and RSSI outputs. RS232 control settings are saved in non-volatile memory for power-on retrieval, so a computer is required only to change a parameter. Trimpots or panel pots can be substituted for the RS232 control of volume and squelch, if desired.

The basic URX-1 design is very flexible, and is able ( with component changes ) to span any 10 MHz band from 100 to 1000 MHz, in 1 KHz steps. URX-1 designs presently exist for 120-130 MHz as well as 140-150 MHz and 465-475 MHz.

Squelch level is adjustable in 256 steps and audio level in 16 logarithmic steps. ( 3 db/step ) Standard channel width is 15 KHz, but a variety of other I/F bandwidths are also available.

Technical Description

The URX-1 is a dual-conversion superhetrodyne with 10.7 MHz / 455 KHz I/Fís, a 10.245 Mhz crystal-tuned 2nd L/O and a PLL-tuned 1st L/O. A 2-pole ( ECS ) 10.7 MHz quartz filter and a 4-pole ( Murata ) 455 KHz ceramic filter set the standard passband to 15 KHz. The front end contains two type BF998 dual-gate FETs, for LNA and 1st mixer. A Motorola type MC3371 I/F chip is followed by a type LM380 audio speaker amplifier. A PIC 16F88 micro provides the RS232 interface. An Analog Devices ADF7012 PLL provides the 1st L/O signal, and an Analog Devices type AD8202 digital pot is used for volume and squelch levels.

Measuring just 1.75 x 3.78 x 0.35 inches and weighing 0.2 ounces, DC power of 8-18 volts with 30 mA "idle" load current, ( squelch closed ) the URX-1 is suitable for a host of situations where light weight, small size, modest power consumption and / or 100% remote control are required.

RS232 Details

The URX-1 RS232 port runs at 4800 baud, 8N1 profile. The following single-keystroke commands ( all terminated with a carriage return ) are included in the URX-1 vocabulary :

Set Recieve frequency

xxxxxx = freq in KHz, e.g. 146565A<cr> = 146.565 MHz

Increase volume one step

Volume is increased by one step
Each step = 3 decibels, 16 steps total
Level 15 = max volume, level 0 = audio "off"

Decrease volume one step

Volume is decreased by one step
Each step = 3 decibels, 16 steps total
Level 15 = max volume, level 0 = audio "off"

Increase squelch one step

Squelch level is increased by one step
256 steps total
Level 255 = max squelch, level 0 = squelch "off"

Decrease squelch one step

Squelch level is decreased by one step
256 steps total
Level 255 = max squelch, level 0 = squelch "off"

Defeat squelch

Squelch level is temporarily set to zero ( squelch = open )
Previous squelch level is saved in RAM to restore it later

Enable squelch

Squelch level is restored to previous value saved in RAM

Auto-set squelch level

Squelch level is automatically set to the present signal level :
Squelch is set to zero level ( squelch = open ) and increased until squelch = closed is detected. This can take a couple of seconds.

( This command should only be used when the channel is clear to avoid "accidentally" setting the squelch level excessively high )

Enable signal reports

Enables RS232 signal reports. Reports are issued whenever squelch changes from "closed" to "open", ( signal = present ) and includes frequency and a 3-digit RSSI number, followed by <cr>. Whenever squelch changes from "open" to "closed, ( signal = absent ) a single <cr> character is sent.

Disable signal reports

Disables signal reports

Enquire reciever status


Causes a status message to be generated and sent by the URX-1.
Status message includes the present reciever frequency, volume level, squelch level, squelch status and RSSI level.

EXAMPLE : 146565,12,137,0,0036<cr>

Present frequency = 146.565 MHz
Present volume level = 12 ( max = 15, min = 0 )
Present squelch level = 137 ( max = 255, min = 0 )
Present squelch status = closed ( 0 = closed, 1 = open )
Present RSSI level = 0036 ( max = 1023, min = 0000 )

The status message does not indicate if the squelch override is enabled or disabled, or if reports are enabled or disabled.


The URX-1 was intended to be "experimenter friendly", and customer modifications ( if done cleanly ) are not a warranty problem. The following "tricks" involve minor changes to the circuit board of the URX-1, which can be performed by the customer, to obtain additional features from the basic URX-1 unit. For specific details about adding any of these modifications, ( or having them installed prior to shipment ) enquire with Bob Simmons.

Panel Pots : The standard URX-1 employs digital pots to allow remote control of audio and squelch levels. If desired, onboard ( 12-turn ) trimpots or external "panel pots" can be substituted for either or both of the the digital pots, although this defeats the RS232 control feature, for that control.

AM Detection : Detection of AM-modulated signals can be achieved by substituting the RSSI signal for the discriminator signal, with some minor circuit changes. The RSSI signal is "fast enough" to follow the modulation envelope of an AM signal, and since 100% AM modulation = 1.76 decibels of carrier power, ( regardless of the actual signal level ) the resulting ( AM ) audio level will be "constant", throughout the entire dynamic range of the receiver. If desired, a user-installed toggle switch can be provided to select between AM and FM modulation for speaker output.

Reciever Bandwidth : The standard BW of the Murata filters is 15 KHz, but the PC board is designed to accept a variety of Murata filter footprints, so other BW filters can be installed, when built. Similarly, the ECS crystal filters are available in different BW values, 30 KHz being the standard one installed in the URX-1.

RSSI Signal : Pin 13 of the MC3371 IC provides an analog RSSI signal that can drive the PicoPlot DF available on this website, or it can be employed otherwise, as the user sees fit.

RF Gain Reduction : The reciever sensitivity can be radically reduced ( over 50 db ) by adjusting the DC voltage on gate 2 of the LNA and 1st mixer MOSFET transistors. The DC sources for these two gates are tied together, ( on the PC board ) to facilitate this modification. This can be useful when the URX-1 is being used as a hunting / DF reciever, ( observing the RSSI signal ) for extreme "close in" hunting, which would otherwise saturate the receiver and drive the RSSI level to its max value.

More Info

Additional information and / or inquiries can be directed via e-mail to Bob Simmons. Detailed schematic diagrams and board pictorials are available ( if requested ) when a URX-1 is purchased, to facilitate customer experiments and modifications. Source code for the PIC micro is not available, but should a customer desire a ( custom ) code change, Iíll try to be accomodating. ( letís talk... )